You can't hammer the same note like 2-2h- but you can do it if it's 2-2h3 or whatever, you just play the note again then hammer on to 3.
Hammering on to an open string if the tab shows --0h2-- then it's just the same as a normal hammer on but the plucked note is an open string. if it's shown --2h0-- then whoever tabbed it is a ******. It should be shown --2p0-- which is just a hammer on in reverse.
Sorry if I'm insulting your intelligence by explaining this. The things tabbed in your post are just ****** tabs posted by morons.
thanks for the responses and yeh it's probally just my interpretation of it. he wrote 0h-5-5-3... and i thought it was hammer 0 then play 553. thanks anyway