[SkepticalDebunker (OP)]

I recently read a comment which said that when people who have diabetes and DID switch to a personality which does not have diabetes, the diabetes can no longer be detected in their bodies, purely because they no longer believe they have diabetes.

Here's the source https://betterbelieveit.net/biology-of-belief-multiple-personality-anomalies/

Another woman, admitted to a hospital for diabetes, baffled her physicians by showing no symptoms of the disorder at times when one personality, who was not diabetic, was dominant. A young man was allergic to citrus fruit in some personalities, but not in others.

Based on how this is worded, it makes it sound like she literally willed away a defective pancreas that doesn't produce enough insulin through faith/belief alone.

F.W. Putnam reported that, out of 100 multiple personality cases he studied, 26 of them displayerd allergies in one personality that were non-existent in another personality. (Putnam et al, 1986).

Allergies, too.

Likewise, I've also read comments saying the placebo effect has cured physical diseases such as cancer and allergies. When these alleged cases are presented, they are always presented as a means of proving the power of mind over matter.

Correct me if I'm wrong, but allergies, cancer, and diabetes are physiological conditions that involve cellular processes not networked by the brain, so how could belief or faith either cure or induce them? What would the mechanism be?

[paul cotter]

No. Diabetes is a "black and white" derangement and it is not accessible to alter through the nervous system. Given the close ties between the immune and nervous systems it may be possible for such to happen with allergies. Note: this is not my area of expertise.

[Petrochemicals]

Diabetes melletus can be caused by other factors than genes, such as auto immune disorders, obesity etc, it's possible that any "cure" is similarly abstract. Linking coincidental events is a path to madness.

The "miracle cure of cancer" is when the body returns to normal and recognises mutant cells and begins to resume disposing of them. It is quite normal, the body not disposing of anomalous cells is the problem.

[alancalverd]

baffled her physicians by showing no symptoms of the disorder .....

The symptoms are not constant and can be modified or suppressed in the short term by diet and possibly exercise. Adopting a different persona will present itself as different behavior, so it is entirely possible that there will be a correlated change in e.g. blood glucose. 

[paul cotter]

Type 2 diabetes can be influenced by one's lifestyle. Type 1 is not amenable to such changes as the β cells in the pancreas have been wiped out and there is no insulin.

[Origin]

To the OPs question:
If a person believes that they do not have diabetes, will their diabetes go away?

The answer is no.