Cant create relations between two tables

Hello!

I have a Task table (from server)   and  Lead (From salesforce), both contain column LeadID.

In the  Lead table, (Lead.LeadID) is  unique and  not in Task (Task.LeadID), so i thought i could use a many to one relationship(or vice versa).

Well, that did not work. So, for every unique entry in  Lead  (as in Lead.LeadID), multiple entries occur in Task.LeadID and each entry has a unique Id (as in Task.Id). 

I do not know what the problem is exactly, perhaps someone could help me out with this. Thanks in advance