Some interphase cells contain twice as much DNA as other interphase cells within the same tissue sample. Is this normal? Why?

Yes, Since the interphase cells in G2 would have twice the DNA as the interphase cells in G1 phase or start of S phase.

Explanation:

Interphase has three stages: G1, S and G2 phase. DNA replication occurs in the S phase and doubles the DNA content of the cell. So, the DNA content of the interphase cells of the same tissue depends on stages of interphase.

The interphase cells which are in the G1 phase has half the DNA content as it is present in the interphase that has entered the G2 phase after completion of DNA replication in S phase.