Great puzzle Emma! I didn't find either of the potential "dupes" you called out to be objectionable at all - I probably wouldn't have even noticed them if you hadn't mentioned it in the clues
Oaf will go to sea for a four-sided mast accessory (CRYPTIC CLUE ALERT)
15A
Doctors and nurses
17A
Unlike most ships
18A
____ angular momentum (I had to slip a physics clue in here)
19A
Opposite of yea
20A
Fed
22A
"The word he's looking for is '____.' ... He better pray he don't find it." (iconic Shirley Bennett line in "Community")
23A
Teacher's favorite
24A
Promo
26A
In ___ (occupied)
27A
"____ diplomacy, the art of telling someone to go to hell and having them look forward to the trip" (play on a Winston Churchill quote that I have seen on a mug before)
29A
Eponyms (this is kind of a dupe of 17-Across but I'm too lazy to change it)
32A
Pass on
34A
Yelling the loudest at the rally, perhaps
35A
"Puh-lease!"
37A
Area in London or Manhattan
38A
"____ by Morning" (George Strait hit)
42A
Compound in 55-Down
46A
Hubble, famously
48A
More likely to say "Please" and "Thank you"
49A
Dot follower
50A
Best, the way the 16-Down might
52A
Opp. WSW
53A
Landing places
56A
Unlike most crossword puzzles (or maybe not and I'm just looking at all the websites for sheltered people?)
57A
Oddly, not cupboards filled with Dasani: Abbr.
58A
Like some motors (I don't know why I put so many nautical clues in this puzzle)
60A
Burdensome
63A
Don't look directly at them!
64A
Merriment
65A
More likely to say "Woe is me!"
66A
Went viral
Verticales
1D
Cause of fever and chills
2D
Hallway carpets
3D
Related on the father's side
4D
"My word!"
5D
Hoover, famously
6D
Prolonged attack
7D
Dad, slangily (ugh, this is an egregious dupe of 20-Across for which I have no excuse except I must've been asleep when I made this grid!)
8D
"p" and "b" are the important ones if you're trying to distinguish "tap" and "tab"
9D
____ Lingus (27-Across carrier)
10D
Some pups
11D
Pelvic bones
12D
Small Japanese ornament
13D
Rice and bamboo, technically
16D
Most wily
21D
"Now!"
23D
Pecan or pumpkin
24D
Culmination of many a degree
25D
Takes back
28D
Word on many bags of candy (seriously, look next time you're in the candy aisle) that I generally ignore
30D
Riddler of ancient Greece
31D
Garlicky mayonnaise
33D
Tweedy's band
36D
Ariel's fishy friend
38D
Prone to allergies (apparently, yes, there is a word for this)
39D
Some wools
40D
Branch of mathematics from the Arabic for "reunite"
41D
Chooses
43D
In the freezer, assuming the freezer isn't broken
44D
Formal disapproval
45D
Is many?
47D
"Well, you get an A for ____" (I always thought that should be an E...)