背 肯 look alikes that aren't alike.

 肯定　　背后
ken3ding4 bei1hou4
certainly behind
背　肯
bei1 and ken3 look alike but are really different both as ideas and in their sounds. the 月　in both indicates one's body, flesh. bear it on your back is bei1 the phonetic clue in 背 is bi３比
There is no phonetic clue in ken3 肯 is a combined idea: the STOPPED 止　person and their BODY　月  what is BEHIND the Body of a Stopped person?

I haven't looked up the etymology of these but even without it we now have good ways to distinguish them.