Check for multiple values in external file prior to vlookup

[Dunro]

Hi everyone,

I have an interesting challenge today.

I'm trying to create a template file that runs a vlookup off of another file. The problem is the other file (which I don't own) can have multiple entries.

I need a way of either flagging that there are multiple entries, OR a sum of all entries.

Template Input:
- Individual's ID (i.e. Person1)
Template output:
- Total amount owing, or a notice saying that this needs to be done manually. (i.e. $60 or "Error. ID not found or multiple values. Recommend manual lookup.")

Amounts Owing:
- Person1, $50
- Person2, $75
- Person3, $25
- Person1, $10


Unfortunately, countif doesn't work with external files (unless they are open). A pivottable could work, but I get wary of storing all the data in each instance of the template.

Are there any other options?

Thanks!