Am I saying it correctly? In doubt about the second "to" 

That second �to � is fine, however, you could also say, �for historians � ... with a slight change in meaning. Using �for � implies an objective view, that is, the general public think that this is important for historians to do,  whereas using �to � means that it is important to the historians themselves. You could turn that round and say �It �s important for historians to make claims based on evidence. � Or, if you are worried about one �to � quickly followed by another, you could say �Making claims based on evidence is important to historians. �