Perhaps people would commonly take them to have the same meaning, and in many contexts they would, but if you look at the words themselves, they clearly don't have to.
1) "It is often considered …"
This means that something is frequently considered. It doesn't indicate how many people are doing the considering:
Between my wife and I it is often considered rude to switch channels without asking the other person first.
This sentence describes two people. I don't think anybody would consider many as something that describes only two people.
2) "It is considered by many …"
This means that a lot of people consider it. But it doesn't say how frequently they consider it.
Once every 100 years, it is considered by many puzzling that there's so much controversy between the new century starting on the year 00 or the year 01.
Something that happens only once every 100 years is certainly not something that can be described by often.
3) In Combination
Because the two phrases can have quite different meanings, it's possible to combine the two in a way that is not simply redundant:
It is considered by many and often that it would be nice to have 3-day weekends.