Q: Does the Planeswalker rule apply when I cast a Planeswalker that another person already has?

A: So Planeswalkers and Legends do not check what any other player has in play. It only cares that you don’t have Planeswalkers and Legends on your side. If you do make a mistake and add a second Chandra to your side, you get to decide which sticks around. This would be important if you have invested turns build Planeswalker Loyalty counters. This how it works when you copy a Planeswalkers and Legends another player has on the Battlefield. So, no problem.

Further: No. You still cannot use Karakas to bounce anything other than a Legendary creature.

This rule has been in effect since 2014.

MrG

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References:

Comprehensive Rules:

704.5j If a player controls two or more planeswalkers that share a planeswalker type, that player chooses one of them, and the rest are put into their owners’ graveyards. This is called the “planeswalker uniqueness rule.”

704.5k If a player controls two or more legendary permanents with the same name, that player chooses one of them, and the rest are put into their owners’ graveyards. This is called the “legend rule.”

Links to other explanations:
[mtgsalvation]…Legend_rule