Dominosa

You can search for pairs that appear only once on the grid:

In this case the pair 0-0 appears only once.

The same way the pair 1-1 appears only once.

Have a look at the upper left corner - the 2 can be part only with the number next to it to there is a dominoe 2-3

Now if you notice the bottom right corner the 2 can only be part of a 2-0 pair. This means that the pair 2-0 cannot be anywhere else on the grid.

You can use the right mouse button to place red borders between numbers which cannot be connected. In this case we separate the other 2-0 pairs.

Now there are some obvious dominoes that can be placed - the 2-2 pair on the top and the 0-3 pair in the middle.

Same goes for the 1-3 on the top-right and the 3-3 on the bottom-left then 1-0 next to it etc.