Twinset by Xanthippe

Xanthippe's offering provided an amusing evening's entertainment. The theme of doubled letters became clear early on with the answer BOOKKEEPER, a word well-known to lexicologists for its three sets of doubled letters. However not until the perimeter was filling in, where some doubles were provided by the end of one word and the beginning of the next, did Gregson realize that the puzzle actually presented pangram of doubles - that is, every possible alphabetic pair was included. There didn't seem to be any very good reason for shifting the clued answers with double letters to other locations, particularly since answer lengths were always given, but perhaps this was just to provide a slight additional hurdle.

Across
9 Seal consumes last of freshwater fish (4)
CARP (Last letter of FRESHWATER inside CAP)
10 Tired clothing artist put back, lacking width (6)
NARROW (RA inside WORN, rev.)
11 Clean vehicle dropping new pal outside university hospital left (6)
VACUUM (VAN minus N(ew) + U(niversity) inside CHUM minus H(ospital))
12 Big sack forming in-flight receptacle for rejected food (7, two words)
SICK BAG (Anag. BIG SACK)
14 Heart of inferior being (4)
ESSE (Middle of LESSER)
15 Citrussy branch chopped on yard (4)
LIMY (LIMB minus last letter + Y(ard))
16 During period Australian returned, skipping one short musical entr'acte (10)
INTERMEZZO (IN + TERM + OZZIE minus I rev.)
18 Uncovered bacon rasher protecting joint round base (5)
BARED (E inside BARD)
Gregson is delighted to learn that his secret turkey-roasting technique actually has a name.
20 Shunts regularly either side of motorway black spot (4)
SMUT (M inside odd letters of SHUNTS)
21 Angler's tip during most severe sea mist (4)
HAAR (First letter of ANGLER inside HARD minus last letter)
23 Hydrocarbon leak exploding round about noon (6)
ALKANE (A(bout) + N(oon) inside jumbler LEAK)
25 European article on illness of the skin (6)
DERMAL (DER (German THE) + MAL)
27 See beyond river bank (4)
RELY (R(iver) + ELY)
29 Miss what might be found in 12, losing volume (4)
OMIT (VOMIT minus V(olume))
31 Wrap special fruit line (5)
SHAWL (S(pecial) + HAW + L(ine))
34 Satisfactory board protected by South African accountant (10)
BOOKKEEPER (OK + KEEP inside BOER)
35 Face is literally relaxed? (4)
DIAL (i.e. LAID back)
36 Check main family branch (4)
STEM (Two meanings)
39 Cases turning up on holiday island become attached (7)
ACCRETE (CA rev. + CRETE)
40 Third of 2 in gallery, Dutch, is flogged (6)
TATTED (Third letter of 2d inside TATE + D(utch))
41 Carol returned without soldier in US army (6)
LEGION (GI inside NOEL rev.)
42 Predatory bird is turning over seabird (4)
SKUA (AUK + (i)S rev.)
A well-known reversal

Down
1 Endless queue by source of Niagara falls (4)
LINN (LINE minus last letter + first letter of NIAGARA)
2 Fine paintings couple take aboard large vessel (7)
ARTWORK (R inside TWO inside ARK)
3 Fellow, Jack, lifting sign (4)
JOEL (J(ack) + LEO rev.)
4 Worked inside fringe record companies (6)
INDIES (Anag. INSIDE)
5 Weather curtailed knock at the crease (4)
RAIN (RAP minus last letter + IN)
6 Strong officer retired after last of the struggles (7)
EFFORTS (Last letter of THE + STR(ong) + OFF(icer) rev.)
7 Info (basic units) audibly describing butt, ferrule and tip? (6)
QUBITS (Homophone CUE BITS)
8 Daughter and son left dim sum cooked with a savoury flavour (5)
UMAMI (Jumble DIM SUM minus D(aughter) minus S(on) + A)
13 Group of countries start to export small instrument (3)
UKE (UK (United Kingdom) + first letter of EXPORT)
17 Millions invested in music company by English woman (5)
EMMIE (M(illions) inside EMI + E(nglish))
19 Delay holding up porter, say (3)
ALE (Hidden, rev. in DELAY)
22 Recognised cutting tool (3)
SAW (Two meanings)
24 Primarily describing ideal nature of paint, not runny maybe (7)
NON-DRIP (Jumble of first letters of DESCRIBING IDEAL NATURE OF PAINT, NOT RUNNY, & lit)
26 Polish dance, a waltz ultimately in dark area (7)
MAZURKA (A + last letter of WALTZ inside MURK + A(rea))
28 Layabout, down, losing weight following time away (6)
LOAFER (LOW minus last letter + AFTER minus T)
30 Ditch fish around river (6)
TRENCH (R inside TENCH)
32 Edge cut narcotic drug (3)
HEM (HEMP minus last letter)
33 Lecturer in widespread disarray (5)
RIFLE (L(ecturer) inside RIFE)
36 Prevent large amount of money heading north (4)
POTS (STOP rev.)
37 Frost covering top of seeds, nuts (4)
MAST (First letter of SEEDS inside MAT)
38 Thickening ground flour with fluid evaporating, unknown amount (4)
ROUX (Jumble FLOUR minus FL(uid) + X (unknown))