After running FastQC on a samples a got an error in the module 'Overrepresented sequences'. And the overrepresented sequence is an adapter: TruSeq Adapter, Index 15 (97% over 40bp).
a) If the sample has too many TruSeq Adapter, Index 15 sequences why FastQC does not through an error in the module 'Adapter content'?
b) In the 'overrepresented sequences' module there are several rows (each with a sequence) saying possible source TruSeq Adapter, Index 15 (97% over 40bp). Some of the sequences in those rows are slightly different from each other. If they are the same Adapter, how can they be different?
c) Should I trimm all of those slightly different sequences or just one of them?