you should never use one key twice, so maybe this is not a best choice.

Yes, this is a (rather well-known) property of the algorithm, which immediately follows from the underlying XORing used.

However, this doesn't necessarily mean it would be a bad choice, as the 'problem' can easily be worked around using some salting mechanism, which would provide one-time keys, while still allowing the same passphrase to be used multiple times.

For example, just compute the key from the passphrase plus some nonce using a one-way hash function such as MD5. The nonce is stored/transmitted together with the data.  (The RC4 algorithm has a few other more subtle problems, but for the moderate security needs as requested here, they're pretty much irrelevant.)