I've searched the topic and found the following two answers.
The best answer suggests to use
booktabs documentation says:
A somewhat technical note: within a
\hline\hlineboth produce a double rule (to allow for page breaks occurring at that point). But the
booktabsrules do not. Longtable’s automatic doubling of
\hlineis questionable, even according to the documentation within that package. But doubled
booktabsrules make almost no sense at all. In the unfortunate event that a
booktabsrule should occur at a page break, then you will have to make the necessary adjustments by hand. (In general, this will mean deleting the oﬀending rule.)
That's not the way it works. I don't want to make any more adjustments by hand since there are already a lot of things to be checked over.
Here the author have faced the problem described above. He'd tried to use
\specialrule from the
booktabs package but the output at page break was buggy. The solution was to use normal
\hline with previously adjusted
\arrayrulewidth. Unfortunately, this can be applied only to the whole longtable, but not to the individual