e09a5875ca rpc: Assert named arguments are unique in RPCHelpMan (João Barbosa)
Pull request description:
Prevents an obvious mistake.
Tree-SHA512: 32c24a1934b17ab6f0d5cd31bdf0388e93ee5156ccc1b4f78eb9fd7f1d4b27a4b978b594ff11812bc9f20987c9fc36bf4497ddaedf18cf6bcbea19c050571334
Backport of Core PR14885