BUG: merge fails to rename when name already exists
Trying to merge a bunch of files in a loop. I realize there's a better way to do this and a workaround, but it took me a second to realize what was going on here.

```
w = pd.DataFrame(np.random.random((4,2)), columns=["x", "y"]) 
x = pd.DataFrame(np.random.random((4,2)), columns=["x", "y"])
y = pd.DataFrame(np.random.random((4,2)), columns=["x", "y"])
z = pd.DataFrame(np.random.random((4,2)), columns=["x", "y"])

dta = x.merge(y, left_index=True, right_index=True).merge(z, left_index=True, right_index=True, how="outer")

dta.merge(w, left_index=True, right_index=True)
```
