Cheapest priced copy (Murders at Karlov Manor): $0.35 at Card Kingdom, near mint, their price list as of .
Due Diligence 
Enchantment — Aura · Common · Murders at Karlov Manor · #14
Enchant creature
When this Aura enters, target creature you control other than enchanted creature gets +2/+2 and gains vigilance until end of turn.
Enchanted creature gets +2/+2 and has vigilance.
Legal inCommanderBrawlStandardPioneerModernLegacyVintagePauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Murders at Karlov Manor | 14 | Common | $0.35 | $0.49 |
One ruling from Wizards of the Coast
- 2 February 2024In the unusual case where Due Diligence's second ability triggers and then Due Diligence becomes attached to the target of its own triggered ability before that ability resolves, when the triggered ability tries to resolve, it won't resolve and none of its effects will happen.