The Government should bring forward legislation that prohibits MPs from undertaking paid work whilst they are serving as an MP. MPs already earn a sizeable salary and have a large workload, therefore they should not require or need additional salaries from other work.
You may be interested in these active petitions
Their primary duty is to their constituents and MPs should be discouraged from having 2nd jobs as their focus should be on their duty as an MP.
However if they chose to have a 2nd job they should not receive payment for it as they already earn a large amount and conflicts of interest may arise if they have other paid work.