It depends on the way the keys are handled. For example using a
symetrical encryption algorithm
using XOR can be really hard to break if only the two people
communicating are using the key and
the key is long enough. The key elements have to be applied in sequence
on the original sequence.
This means that if you have the same character on a number of positions,
chances that it will show
up with the same substitution are slim. There's really no way of
breaking the code using the method
you are trying to use.