Re: Role of comma in this sentence
In full it would read: Someone who both lives in and is eligible to vote in a constituency.
With commas and all the words: Someone who lives in, and is eligible to vote in, a constituency.
Without the repetition of "in": Someone who lives, and is eligible to vote, in a constituency.
"Lives in" must simply mean "resides in". It is only common sense that someone must be alive in order to vote.
In the UK, your permanent place of residence must be the constituency in order to qualify to vote there. If you own, for example, a holiday home where you only stay for a couple of weeks of the year, you are not eligible to vote in the constituency where that house is located. You can only vote in the place that you live for the majority of the time.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.