Creating a unique ID based on and old ID, sequential only between persons (that may have >1 entry)

[Salsalvr]

 I'm looking for a way to create a unique Identifier that will account for duplicates of a client based on an old client ID.  Please see below.

I tried a do loop but it just hung up on me.  Does anyone know code for this. Thanks.

Old _ClientID    NewID

564                         1

564                         1

345                         2

367                         3

321                         4

111                         5

111                         5

111                         5

[novinosrin]

data want;

set have;

by old_clientid;

if first.old_clientid then NewID+!;

run;