Proper words include: book, boom, boon, boor, boot; cook, cool, coop, coot; doom, door; food, fool, foot; good, goon; hood, hook, hoop, hoot; look, loom, loop, loot; mood, moon, moot; noon; pool, poop, poor; rood, roof, rook, room, root; soon, soot; took, tool, toot; wood, woof, wool. There are also some informal or slang words, like boob, coon and toon.
Some six letter words with FF in the middle are:wafflebafflebufferduffermufflepifflesufferduffelpuffindiffer
addassallillofferrbeeseefeepeeteeweezooboocoogoomootoooddebb
Jeep
Some six letter words that end in double letters:accessacrossapogeebamboobefallboo booboohoobungeebypasscaresscoffeecouleecuckoodecreedegreeduressegressembossenrollentreeexcessgoateeharassHawaiilayofflycheemorassmuumuupayoffpeeweepizazzrebuffrecallrecessrefillremissripoffrunoffscrollscruffsetteeshrillsoireesquallstressstrolltarifftattoothrilltoffeetoupeeunlessvoodoo
bookboombootcookcoolcoopcootdoorgoodgoongoophoodhookhooplookloonloomlooplootmoodmoonmoornookpoofpoorpoolrootrookroofroomsoonsoottoottooktoolwoodwoolwoofzoom
Some six letter words with FF in the middle are:wafflebafflebufferduffermufflepifflesufferduffelpuffindiffer
addassallillofferrbeeseefeepeeteeweezooboocoogoomootoooddebb
Some six letter words with OOS in the middle:IgloosKazoosWazoosYahoos
Eel
buffcuffmuffpuffballbellbillbullcallcellmallmillmullnullpillpullrollsillselltilltelltallwallwellwillbassbosscussfusshisskisslessmessmissmassmosslosspasssasstossmuttmittbuttdollfallfellfillfull
Ill and inn.
sully, sunny
Some words that have the letters ME twice in the middle are:amendmentcommencementdismembermentmeantimemeasurementmementomerrimentmesmerisemetronomesometimesummertime
Jeep
Some six letter words that end in double letters:accessacrossapogeebamboobefallboo booboohoobungeebypasscaresscoffeecouleecuckoodecreedegreeduressegressembossenrollentreeexcessgoateeharassHawaiilayofflycheemorassmuumuupayoffpeeweepizazzrebuffrecallrecessrefillremissripoffrunoffscrollscruffsetteeshrillsoireesquallstressstrolltarifftattoothrilltoffeetoupeeunlessvoodoo
bookboombootcookcoolcoopcootdoorgoodgoongoophoodhookhooplookloonloomlooplootmoodmoonmoornookpoofpoorpoolrootrookroofroomsoonsoottoottooktoolwoodwoolwoofzoom
abbreviationaccumulationaccentuationaggressivelyappreciationappropriatedconnotationsdissatisfieddisassociateexpressivelyinstallation