Is it safe to repeat reflow soldering?
There is no generic answer, it all depends on the components involved, let me add a few things to watch out for, and collect for the convenience a few more from the comments.

First of all, read the datasheets of all involved components, what they say about reflow in general and a second reflow possibly.

A lot of things will not be specified in the datasheets though and need to be tested, things to watch out for:


If time passes between the reflows, moisture could be trapped and cause popcorning
Some plastic connectors seem to start decompositing after a second reflow, usually this is not mentioned anywhere and needs to be tried out
Wet electrolytic capacitors may be designed to lose a bit of their electrolyte and vent it during reflow. They would use more than designed for.
Other kinds of caps change values when heated and cooled again, they may go out of spec if heated a second time. Same for other kinds of parts, maybe crystals too.


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thermal stress during heating (while the part is still held by solid solder) might be too  much, especially for MEMS parts and maybe crystals too.
Parts might be held in place by the solder, so watch out for upside down. Also the glue used for holding upside down parts may not be designed for a second heating.


It doesn't seem to make much sense for a manufacturer to specify exactly if a second reflow is possible. In my experience it usually doesn't hurt much, especially if you keep the temperature profile exact.

In the datasheet watch out for any preconditions you might have violated, e.g. if it states the storage temperature prior to reflowing should not exceed 85°C then this might raise an eyebrow about why and if reflowing the first time should maybe count as being store above that temperature.