Cheapest priced copy (Dominaria): $0.35 at Card Kingdom, near mint, their price list as of .
Dub 
Enchantment — Aura · Common · Dominaria · #15
Enchant creature
Enchanted creature gets +2/+2, has first strike, and is a Knight in addition to its other types. (It deals combat damage before creatures without first strike.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dominaria | 15 | Common | $0.35 | $0.49 |
| Core Set 2021 | 16 | Common | $0.35 | $0.49 |
One ruling from Wizards of the Coast
- 27 April 2018Dub can enchant a creature that's already a Knight. It will get +2/+2 and have first strike, but it won't benefit from becoming a Knight.