As far as I can tell, these are the exact same word, down to the cantillation marks. And H121 even says, "the same as H120".
From Strong's:
H120 - 'adam אָדָם, from אָדַם (H119):
man, mankind
- man, human being
- man, mankind (much more frequently intended sense in OT)
- Adam, first man
- city in Jordan valley
and
H121 - 'adam אָדָם, the same as אָדָם (H120):
Adam = "red"
- first man
- city in Jordan valley
Why the two separate entries?