st: Replacing duplicate values

["Pavlos C. Symeou" <p.symeou@lmu.de>]

Dear Statalisters,

I would like to ask for your assistance with the following:

I have a dataset which concerns patents. Every patent is assigned a 
number of International Patent Classifications (IPCs). However, there 
are mistakes in the database and certain IPCs appear more than once for 
a single patent, which is meaningless. Examples are patents with id 6 
and id 7 (ipc_1, ipc_2 etc list the number of IPCs a single patent is 
assigned). For the patent with id 6 we can see that ipc_2 and ipc_3 are 
the same.  Id 7 illustrates a more general issue. Duplicate values may 
not appear sequentially and may appear more than twice.

id    ipc_1    ipc_2    ipc_3    ipc_4
1     A44B    G09F    H04N
2     A47B    G06F    H05K    E05D
3     A47B    G06F
4     A47B    H04N    H05K
5     A47B
6     A47B    F16M    F16M    H05K
7     A47B    A47B F16M A47B

Can you suggest a way to delete the duplicate values, which can be more 
than two, and move the remaining to the left? For example patents with 
id 6 and id 7 would look like this:

id    ipc_1    ipc_2    ipc_3    ipc_4
6     A47B    F16M    H05K
7     A47B    F16M

Best regards,

Pavlos
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