Originally Posted by ArviLaanemets

Your unique numeric entries are entered into column A of sheet Working. And probably you have a header (a string) for this column too.
On some hidden sheet (e.g. Hidden), into some cell (e.g. A1) enter formula, which calculates next unique value, like
Create a Name (e.g. nNextID), which refers to cell with formula
Activate any cell (e.g. A2) on sheet Working, and create a Name (e.g. nCurrID), like
I.e. the Name returns the value of column A of active row in sheet Working. Such Names are referred to as Dynamic Names.
Create a Name (e.g lIDList), like
For column A of sheet Working (for Datarange of this column), create a Data Validation List based on lIDList. Like
=lIDList

Now, whenever you activate this Data Validation List on sheet working, when the cell has any ID value entered previously, this value is only option, and when the cell is empty, the next ID value is the option.

You can also add another IF() level for lIDList, which checks is the active cell in column A, and when not, returns some error message instead ID value.