Little Urgent.....Advice On Logic Flow In SSIS Transformation......!

Dear All,

I have a table A with a KEY column and SSN column.
KEY = 12 digits ( first 3 digits are Department Id , and last 9 digits are SSN)

I have a table B with SSN column only.

both KEY and SSN columns are Primary keys so duplicate entries must be Avoided.

Table A is intended to be popluated weekly from TXT file (SSIS package RUN). I want to achieve somethign like this..!

P-Code sample:

for each Row in TXT file
if TXTfile.KEY = TableA.KEY then
skip and Read/Go to next Row in TXT file
else
INSERT TXTfile.KEY into TABLEA.KEY
SSN_Var = EXTRACT the SSN part (SSNpart.READ)
if SSN_VAR.Exists In TableB.AnyRow then
skip
else
Insert into TableB
End If.
End If
End For Loop.

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Using SSIS controls, what will be best flow and logic to achieve this.....?
any sample scripting code ????

Many Thanks.