Aliasing double → double_(disambiguation)
Link to alias)

Reason:

Vague. There's many double_* tags and it's not immediately clear what it means unless you were to look at the image and examine its tags. Seems to be born of using instead of _. May also be simply duo (because two in the image) or threesome (because double partners, might be a different booru's way of tagging).

(not an exhaustive list)

• double_penetration
• double_dildo
• double_anal
• double_vaginal
• double_cumshot
• double_titfuck
• double_v_sign
• double_oral
• double_hot_dogging
• double_chin
• double_footjob
• double_fisting
• double_anus
• double_knotting
• double_deep_throat
• double_bass
• double_masturbation

Other aliases/implications:

• double_fellatio (I→ or A→) double_oral
• double_cocks A→ hemipenes/diphallism
• double_bun A→ double_buns
• double-breasted A→ double-breasted_suit I→ clothing
• double_bitted_axe A→ double-bitted_axe

EDIT: The tag alias double -> invalid_tag (forum #276826) has been rejected by @Rainbow_Dash.