Why are track stack bars getting joined automatically ?

I made a beat with different elements in different tracks ,1 bar with one pattern and 2nd bar with a little variation and track stacked them.....so basically my pattern becomes of two bars which I now want to replicate, since at some places I might use bar 1 pattern and at other bar 2 , I don't want them to repeat n number of times....and want to manually place them till the end..... now say after bar 1 and 2 if I want to place bar 1 pattern again I duplicate it and place it after bar 2 to realize that now 2 and proceeding duplicated 1 have merged or joined as one track stack, I don't want them to join .......why is it happening and what can I do to keep all bars separate ....... ?