A user cam to me with this problem:
I have 2 email accounts (EmailA & EmailB) and two subfolders (FolderA & FolderB) under my INBOX folder where mail for each account. I'm using rules to move the mail to the correct folder: specific words under sender's (emailA) and recipient's (emailA) address and move it to FolderA. It works well for mail that belongs in the subfolders but my attempts to move mail not addressed to either address results in 2 copies of those messages.
When rules result in duplicates, it always indicates the user is not using the Stop processing action on each rule. My impression is that most users think it means 'stop all rules' so they don't use it. Instead it means 'stop processing this message, move to the next message'.
Rule 1: Move all mail sent to EmailA, to FolderA, and Stop processing
Rule 2: Move all mail sent to EmailB, to FolderB, and Stop processing
Rule 3: Do something with the mail that is left.
The first two rules each move the messages and Stop processing tells Outlook that it should not check the message against any more rules.
Apply more than one Rule to a message
Conversely, if you want to a message to be processed by more than one rule that might apply, you want those rules at the top of the list (since Outlook runs rules in the order they are listed in Rules Wizard), without stop processing selected. Rules that should be the final rule to process a message will have the Stop Processing action.