can I prevent duplicate entries in a column
Of course, you can do it.
Select a sufficiently large range in the right column (for example A2:A500).
Apply a conditional formatting to the selected range:
With the formula
COUNTIF($[First cell]:[Current cell];[Current cell])>1
2. With special condition “duplicated values”
(IMHO, this conditional formatting not will be saved in the final document - when you open the table you will need to redefine it)