I'm on Armory 0.96.5 on hot and cold machines (both Linux).

Bitcoin Core 0.21.0 on hot machine.

1. When trying to broadcast a signed transaction, I first got an error about RPC server not being active. (The transaction file still turned into "_SENT".)
2. I activated RPC with "server=1".
3. I simply tried broadcasting the "_SENT" file again. This time I got this error: "Your Transaction failed to broadcast through the P2P layer"
4. Yet this time, the transaction actually went through!

Therefore my questions:

1. Why did the transaction not go through the first time, but did the second time?
2. What is the meaning of the error and how can I avoid it?
3. Is there anything else to keep in mind in this context? For example, are there any risks in accidentally broadcasting a "_SENT" multiple times?