Codesharing among roads seems to be fairly common in USAnia, especially where old Federal highways have been subsumed by the Interstate system. Which is not immune either; I-80 and I-90 are one and the same for some considerable distance in Ohio and Indiana. Then you have the odd numbering system used by Interstate spurs such that there are (at least) two entirely separate I-580s.
I think the numbering system on the spurs takes advantage of the fact that the spurs are usually far enough apart that it doesn't matter if there are more than one with the same number.
And it seems that the spurs with odd-numbers to start (say, I-105 in Oregon) are dead-ended spurs, while those with even numbers eventually connect back to the "main" road - I-405 east of Seattle, I-205 near Portland, etc.
and of course what is now I-84 used to be I-80N along the Columbia River in Oregon.
I think the doubling-up of I-80/I-90 can be explained by both of them squeezing together south of Lake Michigan and Lake Eire.
Of the numbered "federal" highways, US 20 is rather unique in that it is contiguous. Most of the other cross-continental routes have gaps, but so far, US-20 doesn't. At its west end (Newport, Oregon) there's a sign reading "US 20 East - Boston, MA 3,365 miles" . Not to be mistaken for the 'other' Boston