When an unvowelled mim is immediately followed by another mim, the first is assimilated into the second. Together they are read like one doubled mim with ghunnah. This is idgham shafawi, also called the minor assimilation of identical letters.
«إِلَيْكُمْ مُرْسَلُونَ»
Sent to you. The final mim of «كُمْ» joins the initial mim of «مُرْسَلُونَ».
«لَكُمْ مَا»
What is for you. The two mims are joined in pronunciation.
Do not stop after the first mim and restart with a separate m. Produce a continuous doubled mim with balanced ghunnah and avoid repeatedly opening the lips.