I created Rowe By Rowe - MusicBrainz and their release group and release, then I realized it’s already present as a duo at Release group “Rowe by Rowe” by Tom Rowe & Dave Rowe - MusicBrainz rather than as group with a self-titled album.
Can I cancel these edits? They don’t show as open, but I also don’t see any button that would remove them.
Welcome to MusicBrainz, and thank you for adding this Release!
Also, welcome to the club of “editors who added a Release and then realised that it was already in MusicBrainz”. I have done this myself. And, it looks like it has happened before with Rowe & Rowe!
We don’t like to delete things at MusicBrainz. Each entry has an mbid, each mbid becomes a URL, and each URL could be used in a link somewhere out on the net. Instead, we merge things. Merge the newer, less-referenced thing into the older, more referenced thing.
In your case, that means merge the Release/a26ef which you added into the existing Release/70e75 . See the instructions at How to Merge Releases. And, something needs to happen with the Release Groups. It has been long enough since I did this that I don’t actually know if merging Releases merges the Release Groups also.
One final note: please be sure that the two entries are in fact the same. Do they have the same number of tracks? The same track titles? The same bar code? The same artwork? If any of those differs, then the Releases should stay separate; but you should change the Release Group field for your Release to refer to the older Release Group instead. Then both variations on the Release will be side by side in the Release Group.
So please explore that, and come back with any questions you might have. Good luck!
You also use the same method to merge the two artists. Just make sure you set the direction to push your newly created one into the one that was already here. In this example I’d merge “Tom Rowe & Dave Rowe” into “Tom Row” and any MBIDs that are in use will find a good home. 
Release Groups also merge in the same way. Using the Merge link on the right hand side of the screen.
I am also a member of the club of “editors who added a Release and then realised that it was already in MusicBrainz”. Did one last night. 