It should be possible to build into the design continuous near-field communication; i.e. within the distance of a large solar system. Then a limit on the number of replicating units created per system would be programmed with extreme redundancy controlling replication and evolution (see below).
A small set of units would remain in each explored solar system, repairing themselves and each other, and as other units entered they would signal those units to move on.
If they are designed to NOT evolve, but simply to gather information and make a FEW (say, three) units for dispersal per solar system, then the whole problem goes away--assuming that nothing changes those constraints.
The directive to not evolve could be built in through something considerably beyond today's standard triple redundancy; a kind of robotic super-DNA. Statisticians should be able to come up with a level of redundancy in programming that renders evolution of the design effectively impossible.