Suppose a married woman is able to create a Sheitel that is entirely made up of her own hair (detached, of course). Does that satisfy the requirement for covering her hair?
This is a matter of disagreement among the poskim. Those who say this is an acceptable type of hair covering include Shiltei Giborim (Shabbos 29a in dapei haRif) and P'ri M'gadim (Eshel Avraham OC 75:5).
Those who say it is unacceptable, and differentiate between a regular wig and a wig made out of a woman's own hair, include Ateres Z'keinim (OC 75:3), Be'er Heiteiv (OC 75:8, quoting Ateres Z'keinim), and Magein Giborim (75:8).
CYLOR for a practical ruling.
Mishna Berura (סימן עה, ס' ג, ס'ק טו):
וכתב הפמ"ג דבמדינות שיוצאין הנשים בפיאה נכרית מגולה יש להם לסמוך על השו"ע ומשמע מיניה שם דאפילו שער של עצמה שנחתך ואח"כ חברה לראשה ג"כ יש להקל ובספר מגן גבורים החמיר בזה עי"ש.
The Peri Megadim writes that in those countries were women wear wigs, they can rely on the Shulchan Aruch. One can infer from this, that even one's own hair that was cut off, and then later reattached, one can be lenient (and this counts as a hair-covering). The Magen Giborim was strict in this matter.
(free translation; text copied from here)