Duplicates are kept by default, because in a giveaway they usually mean something: each extra row is an extra chance somebody earned.
When to keep them
If your rules gave extra entries for following, sharing or tagging, the duplicates are the bonus entries. Removing them would quietly cancel the promotion you ran.
When to remove them
If your rules said one entry per person, remove them — but do it before you draw, not after you see the result.
What removal actually does
Removing duplicates keeps the first spelling of each name and adds the chances together. Three rows of @alex become one entry with three chances, so the odds do not change; only the tidiness does. If you want them genuinely reduced to one chance each, remove the duplicates and then turn weights off.
Case sensitivity
Matching is case-insensitive by default, so @Alex and @alex are treated as the same person. That is almost always right for usernames.