Only if:

1) The implementation you are using writes fields in canonical order.  All
official implementations do this, and probably most unofficial ones.
 However, implementations are technically allowed to write fields in any
order.

2) There are no unknown fields in the message.  Your message may have
unknown fields if you originally parsed it off the wire and the sender is a
newer binary that knows about new fields recently added to the .proto file.
 In C++ you can get rid of all unknown fields in a message by calling the
DiscardUnknownFields() method.

If possible, I would recommend designing your application such that it only
requires that equal messages have the same serialization *most* of the time.
 For example, if you were designing a cache where the cache key is the hash
of a serialized message, then the worst that can happen if two equal
messages had different serializations is that you'd perform the same
operation twice rather than hitting cache.  As long as this is relatively
rare, it's no big deal.

As Jon said, this clearly has to be true.  If two messages could have the
same serialization, then how would the parser know which one to produce when
parsing?