bug: heliaInstance.libp2p.addEventListener('peer:discovery', cb) calls cb twice for same peer

[SgtPooki]

Peer discovery should only emit once, unless we are in fact discovering the peer twice via different methods somehow.

Either way, once a peer is discovered, we should cancel any in-progress operations and emit that we discovered the peer only once.

While working on a new helia-script-tag example (via #43), I was able to get the following event log:

1263ms - Created LibP2P instance
1263ms - Created Helia instance
2248ms - Discovered peer QmNnooDu7bfjPFoTZYxMNLWUQJyrVwtbZg5gBMjTezGAJN
2252ms - Discovered peer QmNnooDu7bfjPFoTZYxMNLWUQJyrVwtbZg5gBMjTezGAJN
2252ms - Discovered peer QmQCU2EcMqAqQPR2i9bChDtGNJchTbq5TbXJJ16u19uLTa
2252ms - Discovered peer QmQCU2EcMqAqQPR2i9bChDtGNJchTbq5TbXJJ16u19uLTa
2253ms - Discovered peer QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb
2254ms - Discovered peer QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb
2254ms - Discovered peer QmcZf59bWwK5XFi76CZX8cbJ4BhTzzA3gU1ZjYZcYW3dwt
2254ms - Discovered peer QmcZf59bWwK5XFi76CZX8cbJ4BhTzzA3gU1ZjYZcYW3dwt
74326ms - Connected to QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb
77805ms - Connected to QmcZf59bWwK5XFi76CZX8cbJ4BhTzzA3gU1ZjYZcYW3dwt
135298ms - Disconnected from QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb
138782ms - Disconnected from QmcZf59bWwK5XFi76CZX8cbJ4BhTzzA3gU1ZjYZcYW3dwt

[achingbrain]

Completely agree, but please can you open an issue on libp2p/js-libp2p for this.