Genuine question: is there anything (technically effective) to prevent someone from registering as both a democrat and a republican? And if not, are there any existing how-to guides I could signal-boost?
If you’re in a caucus state, that process might be different; I’m not sure.
Of course, nothing prevents you from being a member of whatever party you want, though there are sometimes laws prohibiting when you can change your registration. Oftentimes, you're prohibited from changing your registered party affiliation within a month or so of the primaries.
You can vote however you want in official elections.
What it affects is which parties' primaries you can vote in.