Rotating Base Language
I have found it very helpful to rotate the base language. When I first started using Duolingo a few years ago, I began by using English as my base language. I "dabbled" with Spanish as the base language, but merely to test it as I had some difficulty switching back and forth (particularly with the mobile app). Since that time, I found out a way to switch back and forth more readily, and zipped through the Spanish to Portuguese tree in a few days and am doing so now with the Spanish to French tree, and so on.
There are some notable benefits to rotating the base language: 1) it reinforces vocabulary in all languages being learned; 2) it increases fluency in the newly learned language ... for example, if you are fluent in English and Spanish ... learning French from both bases will reinforce your knowledge of French more strongly since you are not relying merely on one pathway (i.e., English-French or Spanish-French); 3) it can be beneficial to ESL students also.
I'm sure there are more :-)
I have been working with some younger students (3rd to 6th grades) who are native Spanish speakers by teaching them Portuguese via Duolingo in this manner. Over the last two years, some have been working occasionally with English to Spanish. It is interesting to see how the native Spanish speakers are able to assimilate the similarities of the Portuguese grammar and vocabulary. I've also been helping them to remember substitution patterns that I've used myself that help reinforce vocabulary and lexical similarities. Of course, this works well with languages that share a higher rate of mutual intelligibility.