Hallo Dianne,
as Kiersten described, having the exact same field twice in one custom form is not possible.
But perhaps there is another solution, depending on what you really want.
If, for example you would be able to add exactly the same drop down field twice into a custom form, what would you expect to happen? If the user makes a choice in the first appearing field, should it automatically switch on the second field?
If you want a combination of two fields in your report, you could use to unique fields and combine them in your report column using something like
valueexpression=CONCAT({DE:first field},{DE:second field})
Another approach could be using a calculated field, which "references" the first one, but at the moment I am not sure, if this would work "on the fly", as I did not test it.
So, some more background information can be useful.
Regards
Lars