Luigi Mangione

There are valid reasons to do so, that wouldn't necessarily feel unfair against someone who for sure did something wrong. Basically, it's so that one sticks. Derek Chauvin was charged on second degree unintentional murder, third degree murder, and second degree manslaughter.

These all have different degrees of severity, including average and maximum punishments.

Unintentional 2nd degree is the hardest, and requires a felony (3rd degree assault) to stick. Max penalty 40y, average 12.5.

3rd degree murder requires less; just an "eminently dangerous" act with a "depraved mind" and no regard for life. 25y max, same average

2nd degree Manslaughter is just culpable negligence that can cause unreasonable risk of death or great harm. 10 year max, 4 average.

Now they have to convince a Jury to convict on those charges. If they don't think the felony happened, then the first charge is out. If they don't think he had a "depraved mind", then the second charge is also out. So to make sure he actually gets a punishment, they charge all charges they believe they can get a jury to accept. Charging in both federal and state can also protect against only one politically motivated Governor or president from pardoning all charges, so they would need two pardons.

In addition, if the federal government is worried the state trial might be too biased, they may want their own charges just in case. It seems unfair because you may identify with his actions, but if this was someone lynching people in an extremely racist state (I'd like to say like in the past, but maybe even today) the federal government tacking on charges (in a better admin) could protect against racists just absolving someone from said murder.

In Derek Chauvin's case all charges were successfully convicted, but that isn't always the case. It is a double edged sword though.