Preserve the original order of copyrights in SPDX output

[armintaenzertng]

Description

Based on this comment:

[In SPDX output,] we should not use a set [for copyrights] and instead preserve the original order of copyrights: it matters.

[pombredanne]

FWIW, the order of copyrights does matter as the first one is usually the primary copyright. To keep the insertion order, you can use a list and at last do something like:

def unique(objects):
    """
    Return a list of unique objects.
    """
    uniques = []
    for obj in objects:
        if obj not in uniques:
            uniques.append(obj)
    return uniques

Or do the same on the fly. Or use an ordered set. Or a use a dict using only keys.

[mjherzog]

I agree that we should keep the original order, but there are no standards here. In general the first copyright notice is/should be the most recent but that is different from primary. In many cases the most recent copyright notice is for a minor change.