The first sentence is correct, although as Matt points out, it could be more clear. I would say, "The sales growth in my department is excellent." assuming we mean sales growth and not growth in staff or something else. The second sentence has bigger problems. I would say "Our sales have grown twofold compared to last year." or "Our sales are twice what they were last year". Or if it is the growth that has doubled, as opposed to the actual level of sales doubling, I would say "Our sales growth rate is twice as much as last year's sales growth rate." That may seem like more words than is necessary, but it is very clear (to me anyway). I hope that helps.

It’s understandable but the subjects and objects are a little confusing. The “rate of growth of my department” could mean the people in your department or the sales of your department. Native speakers will generally assume your referring to sales (as headcount follows sales in most situations) but based on context could be something else. A little like 部は… or 部の人は… might be clearer subject but probably not necessary. Also, we typically say “growth rate”; “rate of growth” is more often used in sciences/economics/mathematics to describe the derivative of a change. Consider: “My department’s sales growth is great. We are (we’re) double last year”. Or “Our sales growth has been great, we’re are double last year.”