I have the following strange problem with the GoG downloader:
I bought Rollercoaster Tycoon a few weeks ago and downloaded it.
On two separate occasions since then, I have added a new game to the downloader, and it has deleted my complete Rollercoaster Tycoon download, and started re-downloading that game from scratch without my telling it to, while putting the game I told it to download at the end of the queue.

Is there any solution to this?
It's really annoying that I've had to download Rollercoaster Tycoon 3 times because the downloader keeps deleting it.

BTW, I'm using the Opera browser.

Sometimes downloader starts re-downloading already completed game, when game installer was updated (new version put on servers). The easiest way to avoid it would be deleting completed game from queue. Doing so won't affect files already on HDD.

That suggests Rollercoaster Tycoon's installer has been updated twice in a few weeks, which seems unlikely, but possible I guess.
Completing the third download of that file now, and going to both remove it from the queue and mark the file read-only.

Do you see this problem with other browsers?

Opera has various compatibility issues with all sorts of things for whatever reason. This may be one of them.