Cindy, doubles are notorious for being affected by growing conditions.
In some plant species lower temperatures are considered to increase the proportion of flowers that a double plant produces; in other plant species higher temperatures are considered to increase that proportion. We do not know what conditions change the proportion of double flowers in daylilies.
I call information that is told by one or many daylily grower(s) to others without research evidence, traditional knowledge. Traditional knowledge may be correct but without well designed and analyzed scientific research I do not place much confidence in it.
The first few flowers some double daylily cultivars produce in their first bloom period of the growing season are sometimes single. Later in that first bloom period or in a rebloom period the flowers are double. This has lead to the traditional knowledge (idea) that cool temperatures during flower development cause the flowers to be single and warm temperatures during flower development cause them to be double. It has also been noticed that cultivars that bloom 100% double in Florida may bloom double at much lower rates when grown further north.
However, there apparently are some daylily cultivars that bloom double on their first flowers of their first bloom season and bloom single on later flowers or on rebloom. The doubling behaviour of those cultivars would contradict the idea that warm temperatures during flower development encourage double blooms.
The doubling behaviour of your seedling would also suggest that warmer temperatures do not encourage double blooms. Since no scientific research has been done on how temperature affects doubling in daylilies (and not much scientific research has been done on how temperature affects doubling in other plant species) it is difficult to make any suggestions about what might increase the doubling frequency of your seedling. Perhaps the simplest would be to try moving some of it back into the shady location it was growing in before and comparing the flowering behaviour of the plants growing in the shady location with those left growing in the sunny location.
