There is always the possibility of being on the electoral register twice e.g. students, who can register for both home and term time addresses. In a parliamentary election you can only legally vote once, however. The onus is on you to keep to the rules.
The rules are different in local government elections. The student in the above example can vote in both home and term time local election (assuming a fast enough car!) but not if both addresses are in the same Local Govt area.