Why was this supplier bill refused?

Quick answerIn 2 steps

That supplier's bill number is already entered this financial year: the box says “Already entered on … as …”.
Open that bill to check it is not the same delivery. If it really is a different one, add a suffix to the bill number, like /A.

WherePurchasing → Purchase bills in the back office

Learn it properlyWhy a duplicate supplier bill is refusedOne supplier bill number, once a year — so stock and dues are never counted twice.Open the lesson →

Also asked as

“duplicate bill number”
“bill already entered”