mr Q: According to info in the book "The Messier Objects" (1998) by Stephen O'Meara, M102 is a duplicate observation of M101 by both Messier and Mechain (March,1781 & no date for Mechain's entry). In 1783, Mechain retracted his mistaken observation but Messier didn't after his second observation of M101 (NGC5457). Another galaxy at 11th mag., just 45' from M101 may have been the duplicate both saw but no confirmation on this. Another group of 11th mag. galaxies lie 1 degree north of M101, whose overall pattern resembles that of the nucleus of M101 (another candidate for the duplicate sighting?).
In the end, most sources will claim M102 was a duplicate of M101, by both Messier and Mechain.
What confuses me is your reference to NGC 5866?
Mr Q
Mechain's observational data puts M102 between Iota Draconis and
Theta Bootes. (Some sources have Omricon Bootes, But this may be due to
a misreading of the symbol. They are similar.) Now M101 lies between
Iota Darconis and Ursa Major. Either way the grouping of NGC's
5907,5866 and 5905 lies between ID and TB or ID and OB. Now This is tha
main reason for discounting M101. What puts forward 5866 is it's
magnitude. It's the only one of the three that could be seen by
Messier's scopes. At 10th mag it's equals the faintest Messier objects.
So they could have seen 5866. M101 is some 10 Degrees from 5866. So
Mechain knowing they were close in the sky wanted to make sure and was
unerved by Messier's complete trust in him. I guess he expected Messier
to confim the latest objects.
However both objects should have looked a little different. M101. Messier's discription:
'Nebula without star, very obscure & pretty large, of 6 or 7 minutes
[of arc] in diameter, between the left hand of Bootes & the tail of
the great Bear [Ursa Major].'
Mechain's original discription of M102 is:
'Nebula between the stars Theta Bootis & Iota Draconis:
it is very faint; near it is a star of the sixth magnitude.'
Even the discriptions are different.
The debate goes on.