Update arima.py

[vkverma9534]

Fix duplicated ARMA model entry in stationary_arma_models

[vkverma9534]

Fix duplicated ARMA model entry in stationary_arma_models.

The dictionary had duplicate (p, q) keys which caused earlier entries
to be silently overwritten. This change ensures benchmark correctness.
We need to find an approach so that if the data might be used later we can save it as a dictionary with multiple parameter (as I did) or we remove the ones (in case of redundancy) which will not be used later as dictionary will only use the last assignment as final without even giving a warning.