In CRM 2013, while importing of solution, we faced the error “An item with the same key has already been added”.
A little bit of searching and we found this very good article which describes how to resolve the error – http://nishantrana.wordpress.com/2014/04/15/an-item-with-the-same-key-has-already-been-added-error-while-importing-solution-in-crm-2013/
However as the link suggests, it was not possible for us to delete the field since we had some data where the field was populated in the target environment. Hence we had to take the alternate route.
We unzipped the customizations and looked for the field in the customizations.xml file.
For e.g. in the screen shot below, we could see the field with physical name “new_IsOpportunityCustomer”. However when we went into the customizations of the target environment the name was “new_isopportunitycustomer”. Clearly there was a difference in casing which led to the below error.
To resolve this we changed the physical name in the customizations.xml from “new_IsOpportunityCustomer” to “new_isopportunitycustomer” and re-imported the solution and everything worked fine.
Hope this helps!