Multipath TCP sends packets over multiple paths; multiple different connections, such as cellular and WiFi. It doesn't repeat the packets, unless one of them is dropped. It can be used for several purposes; aggregating bandwidth between the paths, taking advantage of the bandwidth of both connections. It can be used for reliability. And it can be used for transparent handover of connections, where you may walk out of your house and switch over from WiFi to cellular seamlessly.

LWN has a good article on it: https://lwn.net/Articles/544399/