Error "The font contains letters with the same Unicode"

Got the same error in Glyphs.App:
_The font contains letters with the same Unicode. _
Please check the letters: \ nunixxxx = unixxxx.alt1
unixxxx = unixxxx.alt1
I need these .alt Glyphs since they are part of liga Feature:
sub [unixxxx unixxxx ]’ [unixxxx unixxxx unixxxx ] by [unixxxx.alt1 unixxxx.alt1 ];
What am i doing wrong?

But you can only have the Unicode value once

The liga Feature is supposed to combine different Glyphs into a new Ligature. For instance:
sub [unixx01 unixx02 unixx05 unixx06 unixx09 ]’ by [unixx01.alt2 unixx02.alt2 unixx05.alt2 unixx06.alt2 unixx09.alt2 ];
How do i make Glyphs accept these alternate letters?

You can have all these glyphs, no problem, but the Unicode value must be unique. This is also what the error message tells you. Do you know how to set/delete the Unicode value?

This is not a ligature substitution. A ligature substitution is substituting two or more glyphs into one.

Understood, I found doubled a couple of Glyphs using the same Unicodes value and deleted them. Collision solved, thank you @mekkablue.
Is there a sleeker way of finding doublets and setting or deleting the Unicode value than just typing them into the pull-up box on the left bottom?
Where else would i put script that if not into liga, ligature substitution?

Solved that with a lookup:

lookup ligaX {
sub uniXX01 uniXX02 by unXX01.liga;
} ligaX;

You can have Glyphs autobuild your ligature feature. Just name your ligatures appropriately, e.g., x_y.liga For a ligature of x and y. See the appendix of the handbook for details.

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