Return values only if they are not present in current column

I have a column to which I am adding data from another column

I would like to check whether that column has the data that I wish to add. Can you help me with a formula that will tell me whether a cert开发者_开发问答ain value exists in the current column?

Please keep in mind it is a bit more complex than it sounds.

Currently I have this in the cell:

=IF(OR(A3="ETG_C",A3="ETGC"),C3,"na")

I would like to have something like this

=IF(FIND(IF(OR(A2="ETG_C",A2="ETGC"),C2,"na"),K:K,1)," ",IF(OR(A2="ETG_C",A2="ETGC"),C2,"na"))

In other words, if the top value exists already, then add a blank space

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have you tried VLookup?

something like

=if(IsError(vlookup(<what you are about to add>,<column where you are adding>,1, false)),<what you are about to add>," ")