I have a drop-down list that provides input for end-user across a range of
cells. Range is B2:H2 and I want to prevent users from picking duplicate
entries from the drop-down list to populate the range.

Eg.

Drop-down list has 5 entries...A, B, C, D & E
Cell Range is B2:F2
If 1 user chooses A from the drop-down list for cell B2 then the only valid
entries for C2:F2 are B,C,D & E. User # 2 should not be allowed to pick A as
it has already been picked for cell B2.

This is going to be used to help figure out sessions for a technical summit
and ensuring the same session is not picked by multiple attendees which would
cause resourcing conflicts.