Is a ride of a type you already have just as effective as any?

I made a test park with duplicated rides to see if one or the other would be affected negatively.

My test consisted of a simple park (using a recreation of the Forest Frontiers scenario from RCT1) where I placed 2 Merry-go-round rides (and then 3) as well as 2 Twist rides. I also added other rides (Pirate Ship and a coaster) as I went. Duplicates were kept at the same price. Some had different Excitement ratings, often thanks to scenery or proximity to some other ride.

As far as I can tell, duplicates do not have any effect beyond being another ride of the same category (low/med/high intensity). Placing, for example, 2 Merry-go-round rides or 1 Merry-go-round and 1 Haunted House should have a similar effect. Ultimately they're both low-intensity rides and this is what's going to affect attendance.

Admittedly, closing one of the clones seemed to redirect guests to the remaining ride. That said, I believe it is more a consequence of guests looking for non-full queues and similar-intensity rides than anything else.

The best proof may be that I managed to have two nicely running Merry-go-round rides next to each other. The newer one was not always the one doing better, interestingly enough.

As an aside, what seemed to impact how many guests went to a ride the most was:

• The ride location in the park. Merry-go-round at the entrance did poorly for years, in spite of a higher Excitement rating from scenery, while the one in the back kept a full queue
• Price, especially for old rides

I cannot find any specific information on the effectiveness of duplicate rides, however, according to a guide from IGN, reconstructing even one part of a ride will re-boost its effectiveness:

• When nobody wants to go on a ride any more, simply close the ride and delete one piece of the ride, then replace the piece and open the ride again. This can be done as many times as you need (you can also do it when it is raining it will work too). - Source

In the scenario you talk about, obviously there aren't any parts to it, but the principle is the same; demolishing and then reconstructing the merry-go-round in the same spot should restore guests' desire to ride it.