In Parashat Haazinu, it is written "ask your father and he will tell you, your elders and they will explain". I once heard a explanation as to why "ask" is written by one's father whereas by elders there is not written to "ask". Does anybody know? I can't find it.
My own thoughts, and certainly more of a drush than Isaac’s previous answer:
Chazal interpret this passuk as a source for listening to the Rabbis (Shabbos 23a):
מאי מברך מברך אשר קדשנו במצותיו וצונו להדליק נר של חנוכה והיכן צונו רב אויא אמר מלא תסור רב נחמיה אמר שאל אביך ויגדך זקניך ויאמרו לך
What blessing does one make [on the Menorah]? “He Who sanctified us with his Mitzvos and commanded us to light the candle of Chanukah.” Where did He command us? Rav Avya said, from “Do not turn [from what they say, right or left].” Rav Nechemiah said, [from] “Ask your father and he will tell you, your elders and they will say to you.”
Perhaps we can say that there are two types of Rabbinic commandments: those which are in response to something (like Chanukah, which was a response to the Chanukah miracle), and those which are preemptive (like most of their Gezeiros, which are because of a concern). The former is the first half of the passuk: “Ask your father” - i.e. something should happen, and only then “he will tell you” the decree. The latter is the second half of the passuk: with no prompt, “your elders and they will say to you.”
This works well with the parallel Gemara in Sukkah 46a, which learns out Chanukah from just the first half of the passuk:
ומאי מברך ברוך אשר קדשנו במצותיו וצונו להדליק נר (של) חנוכה והיכן צונו מלא תסור ורב נחמן בר יצחק אמר (דברים לב, ז) שאל אביך ויגדך
What blessing does one make [on the Menorah]? “He Who sanctified us with his Mitzvos and commanded us to light the candle of Chanukah.” Where did He command us? From “Do not turn [from what they say, right or left].” Rav Nachman bar Yitzchak said, [from] “Ask your father and he will tell you.”
In a comment on the question, DoubleAA points out that including the verb "ask" only once is consistent with the general pattern in the Torah of not including superfluous words. Had the Torah said "ask" twice where its meaning could have gotten across saying it only once, that would have been noteworthy.
I would add that the single instance of "ask" possibly connotes a unified educational experience, including both the father and the elders. R' Samson Raphael Hirsch's commentary on this verse, Deuteronomy 32:7, describes complementary lessons to be sought from your father and from the elders regarding the history of humanity:
When you ask your father about history, "he will relate it to you" - "ויגדך," which is the same verb as Hagada, the process of telling the Exodus story on Passover. Your father, says R' Hirsch, is meant to "make [human history] vividly realistic to you," presumably employing his special parental understanding of what storytelling style will best capture your interest.
When you then turn to the elders, their job is to "explain" (R' Hirsch's translation here of "ויאמרו") how to apply the lessons we can learn from this history. The elders are particularly well-equipped to make such connections, as they are "gifted with wisdom and insight."
It therefore seems that though you're meant to ask different people for different lessons of and from history, the lessons are meant to complement each other, with the father's stories awakening our interest and conveying the historical data, and the elders' insight helping us glean lessons for our own lives. It's all one educational mission, so it's appropriate to say "ask" once.