How do I remove a manually entered transaction that's now a duplicate of one that came in through my bank feed?

This comes down to whether the bank feed's version of the transaction is still sitting in For review or has already been added as its own separate entry in your books.

If the bank feed transaction is still in For review

Go to All apps, then Accounting, then Bank transactions, select the checkbox next to it, and select Exclude. Since your manual entry already exists and is the one you want to keep, there's no need to bring in a second copy from the bank.

If the bank feed transaction was already added as a separate entry

You now have two real transactions in your books, and one needs to go. Two ways to handle it:

1. Find the transaction that was added from the bank feed, undo it so it goes back to the For review tab, then match it to your existing manual entry instead of adding it again. This is the cleanest fix, since matching links the download to the entry you already made rather than creating a second one.
2. Alternatively, delete whichever of the two entries you don't want to keep. If it's already been reconciled, check with whoever manages your reconciliation before deleting it, since removing a reconciled transaction affects your next reconciliation's beginning balance.

One note on terminology: QuickBooks doesn't have a feature that literally merges two already-posted transactions into a single one. Matching (option 1 above) is the practical equivalent, since it ties the bank download to your existing entry instead of creating a duplicate in the first place. This is the best way to prevent this from happening again if it's a recurring pattern, such as regularly recording a transaction manually before the bank feed catches up to it.

For the full breakdown of finding and removing duplicates depending on where they sit: Fix duplicate transactions in QuickBooks Online bank feeds.