A gene with the sequence ATTCATTCA underwent duplication after a few generations. Which sequence represents this gene after the mutation?

The sequence would be ATTCATTCA.

Explanation:

Usually, when a sequence is duplicated, one of them becomes a non-coding. It's just like a computer. There can only be one specific file, or else a duplicated one will just replace the existing one, if you allow the file to do so. Same with DNA. This is why most of our DNA have no function and why we lost genetic abilities such as being able to produce our own Vitamin C.

So instead of the new gene being ATTCATTCAATTCATTCA, it's simply ATTCATTCA. You simply cannot have duplicates, just like computer files. I wish I could share an amazing video by PBS Eons giving a great explanation about genes and stuff, but sadly doesn't like such stuff, so just search up in ㄚㄖㄩㄒㄩ乃乇 (ebuTouY) "genes we lost along the way" and it should be the first video that pops out. Trust me, it's a great video, it's much better than my explanation.