Runescape: Why are people able to mine over you?

Ok I'm a noob in mining and im trying to mine iron.. But this girl keeps mining over me!!!!! Its making me so angry! Why cant runescape put someone else is mining that just like it says someone else is fighting that?!?!? UGH IM NEVER GONNA GET 60 MINING IF THIS KEEPS HAPPENING PLEASE HELLP!! TY