How to delete duplicate rows in merge replication
The duplicate rows in the MSmerge_tombstone occurs when you restore the database with KEEP_REPLICATION bit.

Its a good practice to configure replication from scratch since you never know that you run into some unknown issue just like you did.

As confirmed by OP - drop the index uc1MSmerge_tombstone, delete the offending row and   recreating the index fixes the problem. 

Remember that you have to be caution when fiddling with MSmerge* tables.