Then let Somnath fight election from Bolpur as independent and re-elected. Then he will be MP of Bolpur.
Since he fought on CPM's ticket using the organization support of CPM, he is a CPM MP first. People voted him not because he is Somnath, but he is a CPM candidate.
If he is not, he should resign from Loksabha and then get re-elected. Only then he deserves to be in Parliament. Otherwise he is disregarding people's mandate.