custom field with checkbox value 'tag' or 'group' causes 'QuickForm Error: element already exists'

Steps to reproduce:
1. Create a custom data group (CDG) with any label (our example: "Language options") with display style "Inline".
2. To this CDG, add a custom data field (CDF) with any label (or example: "Preferred Language"), data type 'Alphanumeric', field type 'CheckBox'.
3. To this CDF, add any number of option values, including one option value with any label (our example: "Tagalog") and the value "tag" (OR the value "group").
4. Edit or create a contact for which this CDG is applied.
5. Observe fatal error: QuickForm Error: element already exists

Possible other ways to reproduce:
I'm guessing that any other existing field name used as a checkbox value will have the same problem; for example, I tried 'first_name' and found that it does yield the same error.

Workaround:
Don't use 'tag', 'group', 'first_name', or any other existing field name as a checkbox value for a CDF appearing in an "inline" CDG (assuming that you know what all those field names are, that future versions don't add a core contact field with a name that collides with one of your custom checkbox values, etc.).