Error running dom_to_df in dev branch

Testing dom_to_df in dev branch gave following error.

```
df <- dom_to_df(dom, exp_type = "z_scores") # Error using function implemented in dev version
Warning: non-unique value when setting 'row.names': ‘Dhcr7’
Error in `.rowNamesDF<-`(x, value = value) : 
  duplicate 'row.names' are not allowed
```