Fix for TicTacToeEnv allows illegal moves #1001

[SanteriVtj]

TicTacToeEnv allows one state to be played multiple times by the same agent. This condition should prevent this behavior and throw an error in case the state is played multiple times.

[chrbauer]

Question: should act! not also call next_player? Or how can i use this env?
The example Pluto notebook is quite broken. (e.g no act! is used)

https://juliareinforcementlearning.org/ReinforcementLearningAnIntroduction.jl/notebooks/Chapter01_Tic_Tac_Toe.html

[jeremiahpslewis]

The test should be an ok example: https://github.com/JuliaReinforcementLearning/ReinforcementLearning.jl/blob/main/src/ReinforcementLearningEnvironments/test/environments/examples/tic_tac_toe.jl