Whoever, being under sentence of imprisonment for life, commits murder, shall be punished with death.

[[Mithu v. State of Punjab : In 1983, Section 303 of the Indian Penal Code (IPC) was struck down. This section provided capital punishment for murder by a person serving a life term in another case. It was held unconstitutional in Mithu v. State of Punjab. The Supreme Court explained that the punishment was not based on rational principle as no judicial discretion was available to a life convict.

A five-Judge Bench of the SC struck down Section 303 of the IPC, holding that the provision violated Articles 14 and 21 of the Constitution. As per Section 303, if a person undergoing life imprisonment committed murder, they would mandatorily be sentenced to death. The SC held that the provision drew an arbitrary distinction between persons committing murder and persons undergoing life imprisonment who committed murder. There was no rationale behind this distinction. Further, a mandatory death sentence prevents courts from exercising their discretion.

(https:legalserviceindia.com/legal/article-8500-case-note-mithu-v-s-state-of-punjab-1983-.html, https:indiankanoon.org/doc/590378/)]]