Use just ... object's id prefixed to make it unique, example for first listing:
@for(cat <- Application.allCategories()) {
<li id="cat_@cat.id">
for second:
@for(prod <- Application.allProducts()) {
<li id="prod_@prod.id">
or if the same product can be displayed in several categories prefix it with cat.id as well:
@for(cat <- Application.allCategories()) {
<li id="cat_@cat.id">
@for(prod <- Application.allProducts()) {
<li id="prod_@(cat.id)_@(prod.id)">