SYSTEM > SELECT LAYOUT BY NAME > choose("name1","name2","name2,"name3")
in this example, the system will select a random layout searching by the name... in this case we have 4 layouts ("name1","name2","name2,"name3") ... the "name2" layout appears 2 times, it means that there is one more chance to the system select the layout name2...
To make it not repeat, you can try to use variables, for example: if the current layout is the name1 , so, the variable layout1 will be = 1 and then you can compare variables and make the things work.
I hope it is not too confuse.