Hi Ted,
try to avoid pasting values that destroys validatation, and
formatting including conditional cell formatting.

You can use this instead, so if you try to change a value above a
cell with the same value you will get your validation error.

Select all of column A with cell A1 as the activecell
=COUNTIF($A:$A,A1)=1


"Ted Rogers" wrote
Many thanks for that - this seems to work if changing a value in a cell but
does not seem to work if adding a value ina blank cell (within the range in
the formula)

Unless I have missed something

Ted