id duplicate due to case sensitive

Dear gurus, 

I am fairly new to Power Bi and I have the following issue.

I am encountering the error "Column CC1ID" in table "T1" contains a duplicate value 121212Oklahoma and this is not allowed on column on the one side of the 1 to M relationship. 

Tracing back the records in the Edit query I have spotted that despite grouping and removing duplicates  i end up wih two entries: 

121212Oklahoma and 121212OKLAHOMA result of a case sensititve problem. Now the problem would be easily  work-arounded by using the functions Lower() Upper() in the editor.

However i wonder if this problem is more due because we are entiring in the World of DAX when try to build the relationship (with consequent case sensitivity thrown in the bin) or other. 

Thanks a lot in advance for your help.

Luca

Any document, links or explanation is more than accceptable as i need mainly to figure out the reason behind this.