Hi GouwsBlom,
Thanks for being part of Apple Support Communities. We'd understand that you're having issues with your Apple Music downloads. We'd like to know more so we can help.
You mentioned that it's letting you download a song that is already downloaded. Are you downloading the same song from the same album or a different one? When you search for the song, are you seeing multiple listings of it in your library? Which iOS are you currently running? You can find that in Settings > General > About. Does this only happen on this current iOS or has it happened on past ones as well?
Is this happening while your device is connected to cellular data, or does this happen while connected to Wi-Fi as well? Test out both to confirm, please: Connect iPhone to the internet
Is this happening when accessing your Apple Music from any other device as well? If you don't have another iOS device, do you have a computer with iTunes, or a Mac with the Music app, so you can test that out?
Keep us updated.
Take care!