5 CFR §839.221 — If my qualifying retirement coverage error was that I was put into FERS by mistake and then, after the error was discovered, I chose my current retirement coverage, can I now make another election?

No, OPM regulations allow certain employees who were put in FERS in error to choose between remaining in FERS or being covered under their automatic retirement coverage. (See § 846.204(b)(2) of this chapter). If you already had this opportunity to choose your retirement coverage; then you may not make an election of retirement coverage based on the same error under these rules.