Lets say you have the following lines:
Team A +130
Team A -1.5 +210
Team B -140
Bet one unit on Team A -1.5 +210 and 1.4 units on Team B.
Team A +130
Team A -1.5 +210
Team B -140
Bet one unit on Team A -1.5 +210 and 1.4 units on Team B.
That's called a polish middle and it'll lose unless you get off numbers on each side.