Help adding ISBLANK to LOOKUP formula

Hi Everyone.

I'm hoping someone can help or point me in the right direction to find the solution. I am using the below formula to extract a list of unique entries to Column D from a range in Column A:

=LOOKUP(2/1(COUNTIF($D$3:$D$3,$A$4:$A$304)=0),$A$4:$A$304)

Sometimes there could be 10 unique entries, sometimes there might be 20 in a dynamic list with space for 30 entries. This leaves cells in my unique entry list that don't get populated with a #N/A..... Unfortunately it is setting my OCD on fire!!

I have tried the following to clean this up and leave the unpopulated cells blank but it gives me a circular cell reference:

=IF(ISBLANK(D3),"",LOOKUP(2/1(COUNTIF($D$3:$D$3,$A$4:$A$304)=0),$A$4:$A$304)) - This was a bit of guessing on my part as to how the formula should look.

I know really I need the formula in another cell and looking more and more at my guess work I'm realising that it needs to perform the LOOKUP logic test first and then leave the cell blank if there are no further unique entries. I thought maybe using IFNA I could achieve this but I'm not sure where to nest this in the formula or the right syntax and structure to do it.

Any help would be greatly appreciated.

Thanks in advance