Copycats

Just Player uses the most permissive license (the Unlicense) so it's perfectly fine the reuse any part of the code without any requirements. However creating duplicate apps without any added functionality or copying app description and screenshots is usually against store (Google Play) policies.

Apps on Google Play

Append package id to https://play.google.com/store/apps/details?id= to get the Google Play link.

com.pankajRana.player.online
com.mee.player // No longer available
com.avrapps.video.player
com.msc.videobuddy // Description
com.fire.android.player // Paid
com.chatur.chaturplayer
com.Pixelsdevelopments.play
com.flixer.player

Other

Source code for sale ($25):

https://www.codester.com/items/33186/Minimal-Video-Player---Android-App-Source-Code