While using onetime encryption is it possible that one can deduce the key of a message if the contents of the message are already known?
Knowing the plaintext and ciphertext will allow you to recover (a portion of) the key.  

The OneTime package referenced in the comments implements a simple one-time pad.  Because the plaintext and ciphertext are related by a simple XOR it is trivial to calculate the portion of the one-time pad that is used for that message.  If all of the rules for using a one-time pad are followed (e.g. no re-use, truly independent random values for each bit, etc), then that portion of the key can be calculated, but it wouldn't provide any value beyond what was already known.