System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce RTX 3090/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 456.71

Blender Version
Broken: version: 2.90.1, branch: master, commit date: 2020-09-23 06:43, hash: rB3e85bb34d0d7
Worked: ?

Short description of error
Two image sequence nodes, referencing the same image files, in the same material, object or scene appear to affect each other.

Exact steps for others to reproduce the error

1. Create a new material
2. Add an image Sequence Node; open and select, for e.g., frame001.png
3. Add another Image Sequence Node and select the same image file.
4. Change one of the node types from Sequence to Single and the other immediately changes.

Two Image Sequence Nodes across and Object or within Scene appear to affect each other differently but I can't tie down the precise behaviour other than to say things stop working as expected. E.g. Setup one Image Sequence Node with a set of frames and then try adding just the first image file using Add | Image | Image As Planes and it will change the playback properties of the previously setup sequence node.