Can a person have bipolar and mdd?. and, why is bipolar considered resistent in treatment?

No: In current psychiatric understanding, a person with depression who's had even one manic or hypomanic episode in the past, would be "bipolar." treatment of bipolar disorder differs from major depressive d/o alone, and includes mood stabilization. Bipolar depression can be difficult to treat because it is more complex; requires care so that you don't induce mania. But it is possible to treat.

No, by definition: Mdd is so-called "unipolar depression", meaning no manic or hypo manic (just less than manic) episodes. If depression occurs first, one may not yet know if the depression is truly unipolar, or one phase of two, the next yet to occur. Bipolar itself is actually not resistant to treatment--but people who suffer from it often are: the mania is often too seductive.