The tools already exist to do this, just not in the form of a merge button. It’s actually useful to be alerted to a tag already existing, and a merger would be irreversible, so it’s probably wise that it’s not just a click away.
You can also just visit a tag page, then click on the icon to the left of Topic which reveals checkboxes to the left of each topic and the following buttons:
Hit Select All on the posts and use Append Tag on the wrench menu:
Or if it’s the only tag on those posts Replace Tag works too.
After that you can delete the unwanted tag.