Does Django ManyToManyField create table with a redundant index?

If I have a model Foo that has a simple M2M field to model Bar:

class Foo(Model):
    bar = ManyToManyField(Bar)

Django seems to create a table foo_bar which has the following indices:

index 1: primary, unique (id)
index 2: unique (foo_id, bar_id)
index 3: non_unique (foo_id)
index 4: non_unique (bar_id)

I recall from my basic knowledge of SQL, that if a query needs to look for conditions on foo_id, index 2 would suffice (since the left-most column can be used for lookup). index 3 seems to be redundant.

Am I correct to assume that index 3 does indeed take up index space while offering no benefit? That I'm better off using a through table and manually create a unique index on (foo_id, bar_id), and optionally, another index on (bar_id) if needed?