1.The [input] file contains more than one line with the same identifier.

This seems to me the only way to obtain the results suggested in the OP. If there were multiple matches of a sub-string from an input file with the contents of the  @isos20 array, I would still expect the input file line to be output only once: grep in a "boolean" context would return true for one or more matches. (Update: I overlooked the fact that Laurent_R had already made essentially the same point here.)

bdorsey: Is there perhaps some further constraint on the content of the input file that you have neglected to mention?

In reply to Re^2: grep of readline matching more lines than elements in array by AnomalousMonk
in thread grep of readline matching more lines than elements in array by bdorsey

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