What knot is considered the most secure for tying a backup knot?

The double overhand knot is recognized for its strength and reliability when used as a backup knot. Its structure consists of two loops that create a snug and secure grip on the primary knot or rope, significantly reducing the risk of slipping or loosening under load. This characteristic makes it particularly effective in scenarios where a secondary point of security is essential, such as in technical rescue operations.

In contrast, the other knots listed may serve specific purposes but do not offer the same level of security for backup applications. The figure 8 knot, while commonly used for primary anchoring due to its strength and ease of untying, is typically not preferred as a backup. The double running whip and overhand knots have their uses but lack the secure grip required for backup safety in rescue situations, making them less reliable choices in this context. Thus, the double overhand knot stands out as the most secure option for tying a backup knot.