Consider reducing image dimension to 512x512 or 512x768. Stable Diffusion is trained on images of that size & it may have difficulty filling the additional space, thereby adding in duplicates or multiples of heads, people, objects, etc.

In addition, try using negative prompts like: duplicate, copy, multi, multiple, two faces, two, disfigured, etc. You can also specify "single" or "one" in your actual prompt.

Keep re-trying generations even if you're not successful at first - it sometimes takes multiple tries.

Note, you're still able to use the 4x upscale feature to generate higher resolution images once you get something you like created.

Lastly, join our Discord server (click your Profile icon and select "Join Discord") for additional help. There's an active community of users who can often help with achieving the desired image and who love to share best practices/tips. We'd flag in particular the #community-help and prompt-tips channels.