A one time match is an indication that they may be the same
individual, but isn't necessarily conclusive. If you get many matches
with some of them near each other in time, or with a static IP
address (an IP address that doesn't change) then you can be more
certain.
Matching IP addresses doesn't work for AOL and some other IPs.
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Elliott Shapiro wrote on vpFREE:
If 2 different individuals have the same originating IP address, may I
assume that the 2 individuals are really one individual, in spite of
different names, etc?