Please take a look at this formula.

=IF(COUNTIF(F$2:F$9,F2)=COUNTIF(F$2:F2,F2),IF(COUN TIF(K$2:K$9,K2)=COUNTIF(K$2:K2,K2),SUMIF(F$2:F2,F2 ,K$2:K2,K2,L$2:L2),"")

There is an error?

I would like to get a sum- if column F and K are identical. The numbers to
be summed are located in Column L.

Example:

F K L sum
s100 species 1 2
s100 species 1 4 6
s101 species 2 1 1
s102 species 3 2
s102 species 3 5 7
....
.....
.....
s1000 species 1 4
s1000 species 1 5 9