Request Collapsing is not same as Request Caching

Though "Request Collapsing" and "Request Caching" sound like close cousins, they are not in same family of functionalities. Both are applicable at transport/server level and do not generally require application level code.

Request Collapsing:

It is useful for cases when you want to collapse (or coalesce) parallel requests, which are same e.g. requesting same resource from the backend, into a single request to the backend. This reduces load on the backend server as number of requests can be reduced significantly. Example of tools that let you do that at http server/entry points are hystrix, varnish etc. You can find more details about request collapsing here.

Request Caching:

It is good for cases when you expect to serve specific type of content frequently and the content isn't expected to change for some period of time. It makes sense to store the value to be returned in a temporary (or persistent) cache store e.g. using a CDN like akamai or letting the browser know that it is ok to cache the value whenever requested, thus reducing load on the backend service. These can be generally governed via HTTP Headers. More details can be found here.

Each use case is different and serves a specific requirements. So it will be possible to mix and match the two when designing your infrastructure.