Tweedledum and Tweedledee
noun
Twee·dle·dum and Twee·dle·dee ˌtwē-dᵊl-ˈdəm-ən(d)-ˌtwē-dᵊl-ˈdē 
: two individuals or groups that are practically indistinguishable
Word History
Etymology
English tweedle to chirp + dum (imitative of a low musical note) & dee (imitative of a high musical note)
First Known Use
1725, in the meaning defined above