Check that id_loan is not duplicated

[jdhoffa]

This relates to a bug identified by multiple users, where projected emission_factors were unreasonably large.

This was a result of the input data having duplicate id_loans per loan. The result was that, while the loan weights was calculated per id_loan, the emission_factor was summed across each loan weight, causing double-counting (sometimes more).

I decided to go the more harsh route, and cause the function to error if input id_loans were duplicated. This will save us headache down the road, as these ids should uniquely identify EVERY loan (otherwise we're gonna have problems).

I also noticed that the emission_factor was still being weighted when by_company = TRUE which doesn't make sense, so fixed that as well.

Maybe closes #160

[maurolepore]

Looks good. I suggested a change and I plan to do it myself.

[maurolepore]

Thanks Jackson, I approve. If appropriate, please remember to update NEWS.md; and to close the relevant issue(s) -- maybe via the commit message when you squash-merge.