Are results of two different tests performed on exact same sample independent or dependent?
For example, if the same group of rats were injected some drug A, results collected, then long enough time waited for them to reach the initial state (before the drug A was taken), then drug B injected and again results collected. Would those results be independent from each other. In my opinion, they are dependent since exact same rats (with exact same physical properties) were used for both tests.
Two rather distinct non-independence issues are a) the carry-over effects of the prior drug and b) two measurements on the same experimental unit. As for (a), it depends on what was measured and whether "long enough time waited for them to reach the initial state" is indeed defensible. Even if it is, it is best to very clearly acknowledge this. Alternatively, this scenario can be modeled using a repeated measures design.
Then, as you rightfully noted, there is still an issue of rats having the same 'sensitivity'(b). This would depend on how the data are analyzed - if the two drugs are two separate research questions, treatments were randomly assigned, and (a) is true, than it may be possible to justify. Certainly not ideal, but has been done. If each set of drug tests had a control and that control was assigned to the same rats, then it gets very difficult to justify.
