Can using the same password lead to the same key?

  1. If two files are encrypted using the same password, does that mean they can have the identical keys? (Assuming the key derivation function is same)

If the key length is constant than collision can happen, right?

  1. How much of a risk it is, to use the same password?

I am new into cryptography so pardon me if I missed something. Or being dumb.