I am no programmer in this field.
We had an addin built on VSTO framework, which works for all Windows Outlook applications. Recently, we built O365 addin and it solves our problem of distribution to Mac. However, the existing Windows Outlook users see two different addins. Is there a way to detect if Outlook windows addin already installed through O365 addin or vice-versa?
Unfortunately, there is no functionality to detect COM/VSTO add-on installation or enumerate installed add-ons from Office.js API. If you have strong business case, you may post request at https://officespdev.uservoice.com/.
On other hand, you are able to check if particular Office.js add-on installed from inside of VSTO/COM add-on. In order to perform this check you would need to know Office.js Add-on ID and this shouldn't be the problem as this is your company product. You would need to enumerate messages in Associated contents table of the Inbox folder by using IMAPIFolder interface. And after find the message with the class “IPM.Configuration.ClientExtension.00000000000000000000000000000000”, where zeros must be ID of your Office JS Add-in. If the message has been found the Office.js add-on has been installed into the user's mailbox.
You may do even more, for example share the add-on properties and settings between VSTO/COM and O365 add-ons.
