The other day, building an SG Base, the entrance portal (the blue swirly column) replicated itself in a room adjacent to the main entrance. It functioned like the main entrance. When I tried to move it into another room, it replicated in the other room but stayed in the original room as well, so at one point I had four of them. The last two gave me "You cannot enter" messages, but the others worked normally. And while they showed up in the room inventory, the "Sell" button was greyed out, so I couldn't delete them.
The next day, went back into the base editor, and the three duplicate portals were now green instead of red, labelled Vanguard portals, and I was able to delete them. Everything has been fine since then.
I am posting it here in case it happens to other people; when I asked for help elsewhere in the forum, and reported the next day that they turned green and were able to be deleted, someone suggested that I should post it as a bug report.