Gernerally, we don't speak "我在说" or "我再说" all alone...there will be a sentence. To be precise, you should understand it with a context. For example, independently , if we say "我在说他的事情" , or "我再说他的事情" , we can't distinguish it. Look at the conversations "你在干嘛?" "我在说他的事情." In this situation, we don't say "我再说他的事情." You comprehend? And for conventional reason, the chinese won't say "我再说他的事情" , we will say "我再说一下(一次)(一遍)他的事情"