MSTs in Christofides Algorithm
The important thing to note is that when merging the MST and the Matching, you create a multigraph. This means even when you merge, even if there are edges that already exist, you add them anyway. So you will always end up with even degree vertices. 

This is step 4 of the algorithm as seen on Wikipedia.

So in your second case, you now have a multigraph where edges $(Somerville \rightarrow Everett)$ and $(Belmont \rightarrow Cambridge)$ each show up twice.