What happens if the runner choses to play [Magnum Opus] twice?
The answer provided by @user30903 in the comments is correct, and I believe your question effectively clarifies what you mean by "playing" as you also reference icebreakers. There is no benefit in installing Magnum Opus twice at the same time: the effects do not stack, and in fact it is detrimental due to the memory usage. The same is true of each individual icebreaker - they do not stack so there is no additional benefit to having two identical ones in play at once.

There is a benefit to having multiple copies in your deck, to raise the chances of drawing it at a useful time.

Similarly there may be benefits to installing it twice during the course of a game, for example if the first copy if destroyed somehow, but not at the same time.

The only (niche) case where I can think it might be useful to install M.Opus or an icebreaker when you already have it in play would be if you were over the hand limit, desperate not to discard any hand cards, and worried that your original copy was going to be destroyed. Then there might be a reason to install a copy, but it is likely to be an expensive solution.

Edit: Thanks to the comments for pointing out several counter-examples of icebreakers where you would want several copies of the same card installed. In general you wouldn't, but there are exceptions. My answer holds true for M.Opus.