You can dupe sign by just breaking fence under "error" sign. Create 2x2 fence and place one extra under one of signs to make rp give error , and then break fence under error sign to make it dupe.

Error means no region is created. Redprotect does nothing until you place a new sign so Redprotect can try aggain to created a region. The error is because you tried to create a region that simoly is to small and the fence are just conecting each other. This kind of duplicate bug would also apear with any sign and fences on your server, if its not caused by a wrong setup of your global worldprotect file.
Also you forget to post used Redprotect Version, may other plugins that your using and your server with Version.
You should also redo your test on a testserver with just Redprotect + Permissionsystem + no other plugins.

When you interact qith the error sign, it already disapear instead break and drop.