Contact Dermatitis: This is a common condition that occurs when the skin comes into contact with an irritant or allergen, leading to a rash. The bilateral distribution on the neck suggests exposure to something like a necklace, clothing, or a chemical, making contact dermatatitis a plausible explanation.

Atopic Dermatitis (Eczema): This chronic condition can cause itchy, inflamed skin and can appear on the neck. It's a common condition and could easily affect both sides of the neck.
Seborrheic Dermatitis: This condition causes scaly patches, red skin, and stubborn dandruff due to an oily scalp. It can also appear on oily areas of the body such as the neck.
Psoriasis: An autoimmune condition that speeds up the life cycle of skin cells, causing cells to build up rapidly on the surface of the skin. The neck is a common area for psoriasis to appear.

Meningitis: Although rare, a rash on the neck could be a sign of meningitis, especially if accompanied by fever, headache, and stiff neck. Meningitis is a medical emergency.
Lymphoma: Certain types of lymphoma can cause skin lesions or rashes, including on the neck. Given the potential severity of lymphoma, it's crucial not to miss this diagnosis.
Infectious Diseases (e.g., Lyme Disease, Syphilis): Certain infectious diseases can cause rashes on the neck among other symptoms. These conditions require prompt diagnosis and treatment.

Pityriasis Rubra Pilaris: A rare skin disorder that causes constant inflammation and scaling (exfoliation) of the skin. It can start on the neck.
Kawasaki Disease: Although more common in children, this disease can cause a rash, among other symptoms, and is important to recognize due to its potential to cause heart problems.
Cutaneous T-Cell Lymphoma (Mycosis Fungoides): A rare type of skin cancer that can cause rash and skin lesions, including on the neck.