Plants are tolerant of gene duplication, possibly related to the fact that their stem cells are permanently active (which is why you can take a branch tip and get it to grow into a whole plant, quite unlike the efforts needed to clone Dolly the sheep). Their development is thus remarkably plastic (so you can get trees at the snowline that look like small shrubs, while the same species grows into tall straight trees a few thousand feet lower). In contrast, gene duplication at a large scale in any animal would probably fundamentally mess up body plan development in non-survivable ways.

Plants might be under active selection for gene duplication since it does allow rapid evolution and facilitates spread into new environments:

Evolution of Gene Duplication in Plants (2016) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4972278/