"If" "then" "else" and "return" command syntax for RPI3 shell script

I am a pipefitter working in a weld shop and need music as commercial radio is very repetitive. I went and bought a RPI3 and put Raspian Stretch Lite on it with the fm_transmitter program. I got the shell script far along enough that the radio works fine but sometimes I'll get a song to play again within a short time. This is just by chance as the RANDOM_FILE variable chooses from an entire directory. I have also gotten the script to log each song that is played in a text file. I would like to use tail and grep to search the text file for the current selected RANDOM_FILE and check if it is present within the last X number of songs in the log. I want the script to restart back at the top defining a new RANDOM_FILE if the RANDOM_FILE is present in the grep command. If it isn't present, I want the script to continue, allowing the RANDOM_FILE to play through the fm_transmitter program.

I would also like for someone to check that the string defining RANDOM_FILE is in fact, truly random and correct.

Below is the script in it's current form. I believe I am close, but am going wrong somewhere as I still am getting repeats. I need help with the "if" "then" "else" and "return" commands. I am hoping this is a quick and easy solution.

Thank you in advance to anyone that takes the time to help me out.

#!/bin/bash

while :
do
    files=(/home/pi/music/*.wav)
    RANDOM_FILE="${files[RANDOM % ${#files[@]}]}"

        if tail -n 25 /home/pi/transmit_log.txt | grep "$RANDOM_FILE" = true ; then
            echo -e "---SONG_SKIPPED---" >> /home/pi/transmit_log.txt ; return 7
        else

#            [ tail -n 25 /home/pi/transmit_log.txt | grep "$RANDOM_FILE" = false ] ; then
            return 22
        fi

    echo -e "$RANDOM_FILE" >> /home/pi/transmit_log.txt
    sox -v 3 "$RANDOM_FILE" -r 44100 -c 1 -b 16 -t wav - | sudo ./fm_transmitter -f 91.7 -
    sleep .5


done