How are UMIs used to dedupulicate in Drop-seq tools?
UMI and barcode are the same but read2 is different, would they be
  collapsed to 1 UMI count or 2?


If the two reads share cell barcodes and UMI sequences and align to the same gene, (even if they are different sequences) then they originate from the same RNA molecule, and should only count as 1.

https://support.10xgenomics.com/single-cell-gene-expression/library-prep/doc/user-guide-chromium-single-cell-3-reagent-kits-user-guide-v3-chemistry