Yes, the extension does not check if there is already an exclamation point. This allow you to add several exlamation points.
The exclamation point is displayed and your recipients will see that your message is important.
The exclamation point added by the Chrome extension will be visible in the subject of the email but the flag important will not be activated in Gmail.
The extension does not modify the header of the email, the X-priority header tag will not be added. That is why the important flag of outlook will not work.
The compose subject have the same id on all winows and for the moment I have no way to control the active window.
This functionnality is available in the pro version of the Mark as important Chrome Extension. It is available in the Chrome Web Store. Get the extension >>