# Why can one double bond exist in some resonance structures but two can't?

I am doing some biology and have not done chemistry in ages. I have two concepts I am struggling with.

1. When drawing Lewis structures, am I right in assuming that the central atom will always require covalent bonds to fulfil its octet before the outer atom's octets are filled? For example when drawing lewis structures, the central carbon atom should be 'given' enough covalent bonds or lone electrons to have a full octet before lone electrons are allocated to the outer atoms in the structure.

2. I do not get why, for example, $\ce{HCO2-}$ can have resonant structures with one double bond (but not two double bonds) between the carbon and oxygens. (please see picture attached)