Duplication across various meta sites is inevitable and okay:
A certain amount of duplication between sites is fine — in my estimation about 10 to 15 percent.
Each site is slightly different and will have different customs, but sometimes, content is duplicated nearly verbatim (I've seen a few more examples) on purpose, with a link back to the original. This is fine, let each meta site control what they need to emphasize to their users — but please continue to include those links, especially back to this meta site as it has much more extensive information and many more eyeballs to keep it updated.
In these cases where a link is added, could the other site automatically include a link back in the "linked" sidebar?
I would think this is a useful feature for the non-meta sites too; though there could be some identity dilution for a fledgling site, and cross-site links are much rarer in the first place, on main sites. However, it seems especially useful given the large overlap of per site metas.