Adding a bit of a correction, it looks like the dupe-check goes a bit further.  It also looks like the only time a particular group/invoice should be returned is if...

It appears to me that this is an attempt at the "simple" deduplication of data (Temp_Work is self joined) combined with a check for valid data (the <> 0 stuff).  That...

After you've done that, automate the creation of a new file/filegroup for a new "next" month and the auto-magic rebuild of the partitioned view.

Another cool part is that, if...

You're on the right track with separate files and file groups. 

If the data is static after being loaded, then base your files and file groups so that...