multiple choices do not work

Install the desktop app of 1Password if you didn't do this before, and do all item management from the desktop app. It's much more feature rich for editing and consolidating items.

What you are offered by the browser extension, are all items matching the website url configured in these items. It's matching the domain part, so if you have one item with "https://login.domain.tld/whatever.html" and another with "https://www.domain.tld/different.html", they both match and you get both items offered. It seems you have multiple items for the same url. You need to consolidate such items to one single item.

In the desktop app, select the "all objects" category in the sidebar on the left. This will get you all items you have. Now identify duplicate items. Sort them by title, in the hope they all have the same or similar titles.

The hardest part is probably to find the one entry with the correct credentials. Delete the others with the same website url, title and user name. The same website url means with the same 2nd level level domain as I mentioned above. All items with "domain.tld" are offered the same, so keep one entry for this domain. It's probably best if you keep the most recently created item and delete older ones, in the hope it contains the most recent url and most recently set password.

Important:
After that, take care what the browser extension will offer if you changed your password or username for a website where you already have a 1Password entry. Most often, it will offer to update an existing item, but sometimes it will offer to create a new item. Read the popup dialog carefully to see if it wants to create a new or update an existing. If you know you want to update the existing item and not create any new, find the existing item and let the browser extension update this entry instead of creating a new one.
If you don't take care, you will start creating new duplicate items again.