...I would assume a badly chosen data sample...
Actually, the OP's example has three sets of duplicate tags: 
Lines  6 -  9: TGATACGGCGACCACCGAGATCTACACTCTTTCC
Lines 15 - 17: TGCTCCGGCGACCACCGAGATCTACACTCTTTCC
Lines 19 - 20: TTCTCCTTCGACCACCGAGATCTACACTCTTTCC

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In reply to Re^3: Memory Efficient Alternatives to Hash of Array by eye
in thread Memory Efficient Alternatives to Hash of Array by neversaint

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