I don't think that the second rule necessarily contradicts the first rule.
We have a setup with five days, and two shifts per day, which could look like this
_ _ _ _ _ _ _ _ _ _
Rule 1 tells us that no students works both shifts on the same day, which means for example that H couldnt work both shifts on monday. So, the following diagram is not allowed
H _ _ _ _ H _ _ _ _
Rule 2 tell us that 2 days in a row, L works the second shift. This could look like the following diagram
_ _ _ _ _ _ _ L L _
And rule 3 tells us that on 2 nonconsecutive days G works shift 1, which mich look like this
_ G _ _ G _ _ _ _ _
The first rule only bans someone from working both shifts on the same day.
