It's not the "message"... but the "signature" that you need to worry about... and chances are that the signature created for an identical message in a different wallet will actually be different (sometimes even if you sign the same message twice in the same wallet) due to the "random nonce" used when creating the signatures.

Message signed in Armory:


Identical message signed in Electrum with identical address/private key, but results in different signature


However, even though the signatures will be different, anyone will be able to verify that it was signed by that address/private key