diff --git a/test/testInsertStatements.js b/test/testInsertStatements.js
index b68e7e6..0912c21 100644
--- a/test/testInsertStatements.js
+++ b/test/testInsertStatements.js
@@ -81,3 +81,85 @@ describe('Test insert() never concatenates statements', function () {
         });
     }
 });
+
+// Issue #304: two rows with the same (id, ts) in one batch - the same datapoint logged from two
+// sources that happened to land on the same millisecond - used to abort the whole INSERT with a
+// primary key violation.
+const DUPLICATE_GUARD = {
+    mysql: /ON DUPLICATE KEY UPDATE/i,
+    postgresql: /ON CONFLICT DO NOTHING/i,
+    sqlite: /ON CONFLICT DO NOTHING/i,
+    // No guard, and that is correct: init() creates `CREATE INDEX i_id on ...(id, ts)`, a plain
+    // non-unique index rather than a primary key, so no uniqueness conflict can arise. Adding one
+    // here to make the dialects look alike would be wrong.
+    mssql: null,
+};
+
+/** The batch from issue #304: ts 1681102802872 appears twice for id 4, from two different sources */
+function duplicateBatch() {
+    return [
+        { table: 'ts_number', state: { val: 54.7, ts: 1681102502871, ack: true }, from: 2 },
+        { table: 'ts_number', state: { val: 54.7, ts: 1681102802872, ack: true }, from: 2 },
+        { table: 'ts_number', state: { val: 54.7, ts: 1681102502853, ack: true }, from: 3 },
+        { table: 'ts_number', state: { val: 54.9, ts: 1681102802872, ack: true }, from: 3 },
+    ];
+}
+
+describe('Test insert() suppresses duplicate (id, ts) rows (#304)', function () {
+    for (const [dialect, sql] of Object.entries(DIALECTS)) {
+        const guard = DUPLICATE_GUARD[dialect];
+
+        it(`${dialect}: ${guard ? 'guards the keyed tables' : 'needs no guard, its index is not unique'}`, function () {
+            for (const table of ['ts_number', 'ts_string', 'ts_bool']) {
+                const [query] = sql.insert('iobroker', 4, [
+                    { table, state: { val: table === 'ts_string' ? 'a' : 1, ts: 100, ack: true }, from: 2 },
+                ]);
+
+                if (guard) {
+                    assert.ok(guard.test(query), `${table} is unguarded: ${query}`);
+                } else {
+                    assert.ok(
+                        !/ON DUPLICATE KEY|ON CONFLICT/i.test(query),
+                        `${table} grew a guard; check whether init() still creates a non-unique index: ${query}`,
+                    );
+                }
+            }
+        });
+
+        if (dialect === 'postgresql' || dialect === 'sqlite') {
+            it(`${dialect}: uses DO NOTHING, not DO UPDATE`, function () {
+                const [query] = sql.insert('iobroker', 4, duplicateBatch());
+
+                // The duplicate sits *inside* one statement. DO NOTHING copes with that; turning it
+                // into DO UPDATE - the obvious way to let the last value win - makes PostgreSQL
+                // raise "cannot affect row a second time" and brings the bug straight back.
+                assert.ok(!/DO UPDATE/i.test(query), `DO UPDATE reintroduces #304: ${query}`);
+            });
+        }
+    }
+
+    it('sqlite: the reported batch really inserts, against a live schema', function (done) {
+        const sqlite3 = require('sqlite3');
+        const db = new sqlite3.Database(':memory:');
+
+        db.serialize(() => {
+            // the real DDL from init()
+            db.run(
+                'CREATE TABLE ts_number (id INTEGER, ts BIGINT, val REAL, ack BOOLEAN, _from INTEGER, q INTEGER, PRIMARY KEY(id, ts))',
+            );
+
+            const [query] = DIALECTS.sqlite.insert('ignored', 4, duplicateBatch());
+            db.run(query, function (err) {
+                assert.ifError(err);
+
+                db.all('SELECT ts, val FROM ts_number ORDER BY ts', function (err, rows) {
+                    assert.ifError(err);
+                    // four input rows, three distinct timestamps - the duplicate is dropped
+                    assert.strictEqual(rows.length, 3, JSON.stringify(rows));
+                    assert.strictEqual(rows[2].val, 54.7, 'the first value for a timestamp wins');
+                    db.close(done);
+                });
+            });
+        });
+    });
+});
diff --git a/test/testMySQLInsertDuplicates.js b/test/testMySQLInsertDuplicates.js
new file mode 100644
index 0000000..4f770e5
--- /dev/null
+++ b/test/testMySQLInsertDuplicates.js
@@ -0,0 +1,125 @@
+const assert = require('node:assert');
+
+// Issue #304 reported a primary key violation on MariaDB/MySQL, so the dialect it was reported on
+// gets an executable check rather than a string assertion. This talks to the server directly - no
+// js-controller, no adapter instance - so it belongs in the MySQL job's `test/testMySQL*.js` glob
+// and costs a second.
+const MySQL = require('../build/lib/mysql');
+
+const HOST = '127.0.0.1';
+const USER = process.env.SQL_USER || 'root';
+const PASS = process.env.SQL_PASS || 'root';
+// Never the real `iobroker` database: this one is created and dropped by the test itself.
+const DB = '__iob_insert_duplicates__';
+
+/** The batch from the issue: ts 1681102802872 appears twice for id 4, logged from two sources */
+function duplicateBatch() {
+    return [
+        { table: 'ts_number', state: { val: 54.7, ts: 1681102502871, ack: true }, from: 2 },
+        { table: 'ts_number', state: { val: 54.7, ts: 1681102802872, ack: true }, from: 2 },
+        { table: 'ts_number', state: { val: 54.7, ts: 1681102502853, ack: true }, from: 3 },
+        { table: 'ts_number', state: { val: 54.9, ts: 1681102802872, ack: true }, from: 3 },
+    ];
+}
+
+describe('Test insert() against a live MySQL (#304)', function () {
+    this.timeout(30000);
+
+    let connection = null;
+    let unreachable = '';
+
+    before(async function () {
+        let mysql;
+        try {
+            mysql = require('mysql2/promise');
+        } catch (e) {
+            unreachable = `mysql2 is an optionalDependency and is not installed: ${e.message}`;
+            return;
+        }
+
+        try {
+            connection = await mysql.createConnection({
+                host: HOST,
+                user: USER,
+                password: PASS,
+                connectTimeout: 5000,
+                // left off on purpose: the adapter never sends batches either, and the test would
+                // stop covering what it is meant to cover
+                multipleStatements: false,
+            });
+            await connection.query(`DROP DATABASE IF EXISTS \`${DB}\``);
+            await connection.query(`CREATE DATABASE \`${DB}\``);
+            // the real DDL from init(), PRIMARY KEY(id, ts) and all
+            await connection.query(
+                `CREATE TABLE \`${DB}\`.ts_number  (id INTEGER, ts BIGINT, val REAL,    ack BOOLEAN, _from INTEGER, q INTEGER, PRIMARY KEY(id, ts));`,
+            );
+        } catch (e) {
+            unreachable = `${e.code || ''} ${e.message}`.trim();
+            connection = null;
+        }
+    });
+
+    after(async function () {
+        if (connection) {
+            await connection.query(`DROP DATABASE IF EXISTS \`${DB}\``);
+            await connection.end();
+        }
+    });
+
+    beforeEach(function () {
+        // Skipping rather than failing keeps `npx mocha test/testMySQL*.js` usable on a machine
+        // without a server. The MySQL CI job always has one, so there it really runs.
+        if (!connection) {
+            console.log(`Skipped: no MySQL at ${HOST} (${unreachable})`);
+            this.skip();
+        }
+    });
+
+    it('stores a batch that contains the same (id, ts) twice', async function () {
+        await connection.query(`TRUNCATE \`${DB}\`.ts_number`);
+
+        const [query] = MySQL.insert(DB, 4, duplicateBatch());
+        await connection.query(query); // used to throw ER_DUP_ENTRY
+
+        const [rows] = await connection.query(`SELECT ts, val, _from FROM \`${DB}\`.ts_number ORDER BY ts`);
+        assert.strictEqual(rows.length, 3, `four rows, three distinct timestamps: ${JSON.stringify(rows)}`);
+        assert.strictEqual(Number(rows[2].ts), 1681102802872);
+        assert.strictEqual(rows[2].val, 54.7, 'the first value for a timestamp wins, the duplicate is dropped');
+    });
+
+    it('would fail without the guard - this is the bug that was reported', async function () {
+        await connection.query(`TRUNCATE \`${DB}\`.ts_number`);
+
+        const [query] = MySQL.insert(DB, 4, duplicateBatch());
+        const unguarded = query.replace(/ ON DUPLICATE KEY UPDATE id=id/i, '');
+        assert.notStrictEqual(unguarded, query, 'precondition: the guard was in the statement');
+
+        await assert.rejects(
+            () => connection.query(unguarded),
+            err => {
+                // exactly what issue #304 shows: Duplicate entry '<id>-<ts>' for key 'PRIMARY'
+                assert.strictEqual(err.code, 'ER_DUP_ENTRY', err.message);
+                assert.ok(err.message.includes('4-1681102802872'), err.message);
+                return true;
+            },
+        );
+    });
+
+    it('still reports a real error instead of swallowing it', async function () {
+        await connection.query(`TRUNCATE \`${DB}\`.ts_number`);
+
+        // The guard is `ON DUPLICATE KEY UPDATE id=id`, not `INSERT IGNORE`, so that only the
+        // uniqueness conflict is suppressed - see the comment in src/lib/mysql.ts. An INSERT naming
+        // a column that does not exist has to keep failing.
+        await assert.rejects(
+            () =>
+                connection.query(
+                    `INSERT INTO \`${DB}\`.ts_number (id, ts, nope) VALUES (1, 2, 3) ON DUPLICATE KEY UPDATE id=id;`,
+                ),
+            err => {
+                assert.strictEqual(err.code, 'ER_BAD_FIELD_ERROR', err.message);
+                return true;
+            },
+        );
+    });
+});