# Remove duplicated sequences from an alignment

The `omit_duplicated` app removes redundant sequences from a sequence collection (aligned or unaligned).

Let’s create sample data with duplicated sequences.

Creating the `omit_duplicated` app with the argument `choose="longest"` selects the duplicated sequence with the least number of gaps and ambiguous characters. In the above example, only one of `c` and `d` will be retained.

Creating the `omit_duplicated` app with the argument `choose=None` means only unique sequences are retained.

The `mask_degen` argument specifies how to treat matches between sequences with degenerate characters.

Let’s create sample data that has a DNA ambiguity code.

Since “Y” represents pyrimidines where the site can be either “C” or “T”, s1 indeed matches s2 and one of them will be removed.

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Filter sequence collections and alignments by length