Diplopia can also occur when viewing with only one eye… multiple image perception includes the consideration of such conditions as … diffraction-based (rather than geometrical) optical models have shown that common optical conditions, especially astigmatism, can also produce this symptom. - wikipedia

Monocular diplopia may be induced in many individuals, even those with normal eyesight, with simple defocusing experiments involving fine, high-contrast lines.

In this sense, double vision is neither dangerous nor harmful, and may even be enjoyable. It makes viewing stereograms possible.

see also

• I’ve acquired a new superpower / HN - fast find difference between two images