From this picture only it will be hard to tell I'm afraid.

a few things anyway: if the two halpotypes would be present in your assembly the total assembly size should be nearly double as what you would expect. I assume the genome size you mentioned is the haploid size? (== 1C value) .

It could indeed also very well be that this particular genome is (recently or well retained) duplicated, that will also give a similar BUSCO result. Is there any other indication of duplication? (eg Ks value analysis or such). If you would make this plot for the poplar genome I believe it will be similar to what you present here. If you are working with haplotypes there is of course a thin line between allelic regions and duplicated regions.