The Flink documentation says "The only relevant information that is set on the result elements is the element timestamp [...] which is [set to] end timestamp - 1 [...]"
In case I have to consecutive event time windows, where the first one is a tumbling window with a size of 5s and the second one is a sliding window with a size of 5s and a slide of of 1s, does this mean the second window will emit 5 times the same values within an aligned 5s event time period (assuming no late events)?
If you have a 5-minute sliding window consuming the output of a 5-minute tumbling window, then the sliding window will only see as input a burst of events coming every 5 minutes, when the tumbling window emits its results -- all of which will carry the same timestamp.