Duplicate Sent Items Even thought the box is checked to not save

[CBuongio]

Fairly recently my Outlook 2016 (through Office 365) has started duplicating all of my sent items.  I use a Gmail account and have followed the steps as instructed for outlook, as in the box has already been checked to not save sent items - but it doesn't matter, still does.  I am using Windows 7 Professional, which I understand is no longer supported - does this impact the operation? Any suggestions would be helpful.

Thank you

[#2]

@CBuongio The OS shouldn't have an effect.

When you checked the setting in account settings, did you untick 'Don't save copies of sent items', save and exit the dialog then go back in and re-enable it?  If that doesn't fix it,  turn off the option to 'Save copies of messages in the Sent Items folder' in File > Options > Mail, do you still get duplicates?

[#3]

@Diane_Poremsky_MVP 

Thank you for the reply, I ended up doing an On-line repair to the Office 365 and that seems to have corrected it, I had previously tried the quick repair with no luck.  Hopefully it will not start up again, if it does I will try your suggestion.

[#4]

@CBuongio I have the same issue, mine was already unticked from resolving the issue previously, but it's come back today, are you able to provide the link for the repair tool?

[#5]

@ilonka65  it's on the help menu in outlook or can be downloaded here - https://aka.ms/SaRA-OutlookAdvDiagnostics 

More info on it here - 

Scan by using Microsoft Support and Recovery Assistant - Outlook | Microsoft Docs

[#6]

@ilonka65  oops - that was for SaRA - the office repair option is accessed in Windows Settings. 

To repair your Office installation, right click on the start menu and choose Apps & Features, find and select the Office installation. Choose Modify then select the desired repair option.  In this case, try the online repair option. 

[#7]

I have the same issue, but with the Outlook Account itself. 

a) I have configured Outlook with MS 365 in the standard (MS 365 way) - this works o.k.

b) Using the same MS365 account with a third party application, such as an CRM and with the IMAP Settings provided by MS for accessing this account.

All mails sent view the CRM (b)) show up twice in Outlook.  The option "do not save in sent folder" does not really work, as MS does let you change this option only in Outlook Rich client not in OWA. Additionally, I do want mails, sent via Outlook Rich Client "to be stored" in Sent folder too. 

A test with another account showed, the behavior for other (than MS) IMAP servers is the same, than for Microsoft itself. I have configured the MS365 as IMAP account in Outlook (instead of MS365), the mails showed up twice. With this setup (IMAP) the option of deselecting "save messages in Sent" folder can be deactivated. 

Looking in other forums I have not really been able to find a solution. Only workarounds to delete duplicate items. 

[#8]

I'm not sure how the CRM using IMAP affects the web client - IMAP doesn't use the web client., it uses the SMTP service.

The problem is the CRM app's SMTP service doesn't allow you not to save sent items.... and when you send mail using smtp, the server will save a copy of the sent item. This is a fairly new feature - I first noticed it maybe 6 months ago. As far as I know, there isn't a way (using PowerShell or the admin UI) to disable this.