Attributes like Flat can affect not only evaluation, but also operations such as pattern matching. If you give definitions or transformation rules for a function, you must be sure to have specified the attributes of the function first.
Here is a definition for the flat function f:
In[3]:= f[x_, x_] := f[x] https://wolfram.com/xid/0m5dyglvgmwz4-si8
Because f is flat, the definition is automatically applied to every subsequence of arguments:
In[4]:= f[a, a, a, b, b, b, c, c] https://wolfram.com/xid/0m5dyglvgmwz4-iw6
Out[4]= f[a, b, c]
In[1]:= SetAttributes[f, Flat]
Out[1]= None
In[2]:= f[f[a, b], c]
Out[2]= f[a, b, c]
In[3]:= f[x_, x_] := f[x]
Out[3]= None
In[4]:= f[a, a, a, b, b, b, c, c]
Out[4]= f[a, b, b, b, c, c]
Following paragraph is taken from https://reference.wolfram.com/language/tutorial/EvaluationOfExpressions.html.en?source=footer
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