There are 3 instances where this can occur i think:
1 - You don't disconnect properly and initiate a new connection because of a power failure or some-such. This is the reason that most ISP's allow at least 2 concurrent connections as it takes quite a while for the old connection to self-disconnect, but still using the same telephone access point.
2 - You are using a dial-up connection to initiate the PPPOE connection form within your network, and are initiating more than one dial-up connection at the same time, but still using the same telephone access point.
3 - You and somebody else are connecting at the same time to the same PPPOE account from different telephone lines.
All of these are perfectly normal and allowed unless:
Instance number 3 will not be allowed if the ISP is providing an uncapped solution, but that's usually the ISP's problem, and not yours.