If you understand shiny and reactivity well, you will know that a reactive expression gets re-run whenever any of its reactive dependencies are invalidated. This is generally the desired behaviour, but there is one caveat that comes up occassionally: even if the underlying value of the reactive dependency hasn't changed, it can still be considered "invalidated". This can mean that a reactive expression will run again with exactly the same values because its dependencies have not changed, even though they're invalidated. Joe Cheng (author of shiny) has a solution for this.
Define a dedupe function as follows:
dedupe <- function(r) {
makeReactiveBinding("val")
observe(val <<- r(), priority = 10)
reactive(val)
}
And now you can wrap any reactive(...) expression with dedupe(reactive(...)) so that it will only get invalidated if the actual value changes.