Nostr: Can I use the same identity in multiple Nostr clients?

Short answer

Yes. Clients can use the same public identity when they can sign with the corresponding private key or an authorized signer. Posts and follows may appear differently because clients support different NIPs and connect to different relays. Avoid typing your nsec into every website; a browser or remote signer can reduce how many applications directly handle it.

Sources

NIP-01: Basic protocol, NIP-07: Browser signers, NIP-46: Remote signing.