Final answer:
When a lab receives a test tube with double labeling, it indicates that the test tube contains two different labels. This is often used in experiments such as DNA sequencing, where each fluorescent label represents a different base in the sequence.
Step-by-step explanation:
When a lab receives a test tube with double labeling, it means that the test tube contains two different labels. This could be used in various experiments where different chemical procedures are performed on the labeled molecule. For example, in DNA sequencing, each of the four dideoxynucleotides carries a different fluorescent label, and the sequence is read from a laser scanner. This allows the detection of even a single base difference in length.