time step to equal time point when using mat-file?

I use an mat.file created by a matlab program as sourse in a simulink program. This mat-file gives me data in the time points 1,2,3,4,... this data goes into an add-block, being added with its own result. so it is basically a simple sum function. My problem now is, that it does 'add' way to often. I would like it to add only once per time point, but the time steps in the add function are way smaller, so that it adds the input value several times each.
How can I get it to add only once per time point?