Using MATE desktop I get two icons for Telegram in the Applications > Internet menu:
clicking either of these icons launches the application (and if I click either one while telegram is running, it is unminimised and/or gets focus)
I installed Telegram Desktop by this method:
sudo add-apt-repository ppa:atareao/telegram
sudo apt-get update
sudo apt-get install telegram
It works perfectly (& I love it) but I only want one icon... How can I make one of them disappear?
In MATE, right clicking on the menu and selecting "Edit menus" will get you the menu editor:
Find the Telegram entries under the Internet category in the menu editor application, and uncheck one of them. The process is very similar for XFCE. Note that using this menu editor, you can completely customize the menu, move things around, and so on. It's pretty cool :-)