I'm working on uploading my video but I'll explain it as best as I can. I tried this on tourney realm and on live and they both work.
Step 1
Remove two of your current glyphs (make sure the 3rd one isn't the one that you want to have two of.)
Step 2
Go to the Lexicon of Power with the glyphs that you want to put in.
Step 3
Put the first glyph you want in, and chain cast the second one (when there is about .1 seconds left on the first glyph installing click to install the 2nd glyph.)
If you have done this correctly you will have 2 of the same glyph!
If you have done this incorrectly however it will say something along the lines of you already have the glyph inscripted in your spell book or something
Good luck and I will have the video up soon!